Add flashboard target to the stmhal Makefile to run dfu-util
Add flashboard target to the stmhal Makefile to run dfu-util

Which allows you to run "make -C stmhal flashboard" from the top-level directory.
Also ties in nicely with my configurable BUILD directory from #506
methoxid/micropystat,HenrikSolver/micropython,neilh10/micropython,HenrikSolver/micropython,bvernoux/micropython,misterdanb/micropython,PappaPeppar/micropython,alex-robbins/micropython,puuu/micropython,swegener/micropython,cloudformdesign/micropython,paul-xxx/micropython,praemdonck/micropython,xuxiaoxin/micropython,lbattraw/micropython,ganshun666/micropython,mianos/micropython,feilongfl/micropython,pramasoul/micropython,TDAbboud/micropython,SHA2017-badge/micropython-esp32,turbinenreiter/micropython,matthewelse/micropython,vriera/micropython,supergis/micropython,xhat/micropython,warner83/micropython,galenhz/micropython,pfalcon/micropython,chrisdearman/micropython,ChuckM/micropython,omtinez/micropython,praemdonck/micropython,kostyll/micropython,danicampora/micropython,utopiaprince/micropython,tobbad/micropython,lbattraw/micropython,SungEun-Steve-Kim/test-mp,bvernoux/micropython,kostyll/micropython,orionrobots/micropython,utopiaprince/micropython,rubencabrera/micropython,lowRISC/micropython,matthewelse/micropython,vriera/micropython,slzatz/micropython,dinau/micropython,noahchense/micropython,aethaniel/micropython,omtinez/micropython,MrSurly/micropython-esp32,methoxid/micropystat,jimkmc/micropython,tdautc19841202/micropython,methoxid/micropystat,feilongfl/micropython,feilongfl/micropython,SungEun-Steve-Kim/test-mp,danicampora/micropython,dmazzella/micropython,rubencabrera/micropython,swegener/micropython,xhat/micropython,dhylands/micropython,bvernoux/micropython,tdautc19841202/micropython,vitiral/micropython,adafruit/micropython,martinribelotta/micropython,oopy/micropython,oopy/micropython,dxxb/micropython,turbinenreiter/micropython,redbear/micropython,feilongfl/micropython,chrisdearman/micropython,cloudformdesign/micropython,pfalcon/micropython,trezor/micropython,redbear/micropython,Vogtinator/micropython,ericsnowcurrently/micropython,bvernoux/micropython,aethaniel/micropython,cnoviello/micropython,oopy/micropython,adafruit/micropython,vitiral/micropython,cloudformdesign/micropython,ceramos/micropython,micropython/micropython-esp32,hiway/micropython,ryannathans/micropython,xyb/micropython,mgyenik/micropython,tralamazza/micropython,mianos/micropython,noahchense/micropython,Vogtinator/micropython,cnoviello/micropython,dmazzella/micropython,pfalcon/micropython,lowRISC/micropython,drrk/micropython,micropython/micropython-esp32,xhat/micropython,misterdanb/micropython,ryannathans/micropython,stonegithubs/micropython,cloudformdesign/micropython,blazewicz/micropython,blmorris/micropython,EcmaXp/micropython,ganshun666/micropython,dinau/micropython,EcmaXp/micropython,jlillest/micropython,alex-march/micropython,hiway/micropython,cnoviello/micropython,Peetz0r/micropython-esp32,omtinez/micropython,Timmenem/micropython,matthewelse/micropython,orionrobots/micropython,noahwilliamsson/micropython,chrisdearman/micropython,vitiral/micropython,hosaka/micropython,jlillest/micropython,xuxiaoxin/micropython,trezor/micropython,SungEun-Steve-Kim/test-mp,ahotam/micropython,redbear/micropython,pfalcon/micropython,turbinenreiter/micropython,tobbad/micropython,kerneltask/micropython,blazewicz/micropython,supergis/micropython,paul-xxx/micropython,SHA2017-badge/micropython-esp32,dxxb/micropython,puuu/micropython,pozetroninc/micropython,adamkh/micropython,ChuckM/micropython,ruffy91/micropython,suda/micropython,kostyll/micropython,redbear/micropython,infinnovation/micropython,MrSurly/micropython,mpalomer/micropython,adafruit/micropython,ernesto-g/micropython,alex-march/micropython,tuc-osg/micropython,martinribelotta/micropython,deshipu/micropython,ganshun666/micropython,vitiral/micropython,warner83/micropython,slzatz/micropython,pramasoul/micropython,pozetroninc/micropython,AriZuu/micropython,mpalomer/micropython,paul-xxx/micropython,heisewangluo/micropython,kerneltask/micropython,drrk/micropython,galenhz/micropython,pramasoul/micropython,cnoviello/micropython,heisewangluo/micropython,lbattraw/micropython,adamkh/micropython,selste/micropython,hosaka/micropython,dxxb/micropython,blmorris/micropython,blazewicz/micropython,ChuckM/micropython,infinnovation/micropython,hiway/micropython,kerneltask/micropython,alex-robbins/micropython,methoxid/micropystat,PappaPeppar/micropython,praemdonck/micropython,dhylands/micropython,aethaniel/micropython,tuc-osg/micropython,selste/micropython,pozetroninc/micropython,SHA2017-badge/micropython-esp32,kostyll/micropython,alex-march/micropython,galenhz/micropython,martinribelotta/micropython,pfalcon/micropython,henriknelson/micropython,Peetz0r/micropython-esp32,adamkh/micropython,mhoffma/micropython,chrisdearman/micropython,supergis/micropython,Vogtinator/micropython,cwyark/micropython,noahwilliamsson/micropython,orionrobots/micropython,supergis/micropython,rubencabrera/micropython,firstval/micropython,SungEun-Steve-Kim/test-mp,mpalomer/micropython,martinribelotta/micropython,tdautc19841202/micropython,firstval/micropython,swegener/micropython,ernesto-g/micropython,warner83/micropython,tuc-osg/micropython,henriknelson/micropython,vriera/micropython,lbattraw/micropython,blmorris/micropython,ruffy91/micropython,utopiaprince/micropython,puuu/micropython,puuu/micropython,mianos/micropython,ganshun666/micropython,mhoffma/micropython,Timmenem/micropython,noahwilliamsson/micropython,selste/micropython,galenhz/micropython,alex-robbins/micropython,AriZuu/micropython,vriera/micropython,selste/micropython,trezor/micropython,ahotam/micropython,mhoffma/micropython,jmarcelino/pycom-micropython,mgyenik/micropython,kerneltask/micropython,xuxiaoxin/micropython,jimkmc/micropython,ernesto-g/micropython,trezor/micropython,xyb/micropython,omtinez/micropython,deshipu/micropython,ahotam/micropython,pozetroninc/micropython,dmazzella/micropython,misterdanb/micropython,noahwilliamsson/micropython,drrk/micropython,deshipu/micropython,dxxb/micropython,aethaniel/micropython,PappaPeppar/micropython,skybird6672/micropython,TDAbboud/micropython,redbear/micropython,adamkh/micropython,ryannathans/micropython,KISSMonX/micropython,PappaPeppar/micropython,tdautc19841202/micropython,supergis/micropython,torwag/micropython,adafruit/circuitpython,EcmaXp/micropython,dinau/micropython,toolmacher/micropython,stonegithubs/micropython,xuxiaoxin/micropython,skybird6672/micropython,stonegithubs/micropython,MrSurly/micropython,suda/micropython,dhylands/micropython,ahotam/micropython,kerneltask/micropython,torwag/micropython,heisewangluo/micropython,alex-march/micropython,cwyark/micropython,dhylands/micropython,tdautc19841202/micropython,torwag/micropython,ganshun666/micropython,skybird6672/micropython,micropython/micropython-esp32,HenrikSolver/micropython,ChuckM/micropython,TDAbboud/micropython,neilh10/micropython,alex-march/micropython,bvernoux/micropython,rubencabrera/micropython,utopiaprince/micropython,toolmacher/micropython,ruffy91/micropython,toolmacher/micropython,KISSMonX/micropython,dinau/micropython,SHA2017-badge/micropython-esp32,lbattraw/micropython,tralamazza/micropython,jimkmc/micropython,SungEun-Steve-Kim/test-mp,emfcamp/micropython,heisewangluo/micropython,matthewelse/micropython,methoxid/micropystat,tobbad/micropython,praemdonck/micropython,HenrikSolver/micropython,stonegithubs/micropython,pramasoul/micropython,MrSurly/micropython-esp32,Peetz0r/micropython-esp32,orionrobots/micropython,EcmaXp/micropython,hiway/micropython,TDAbboud/micropython,jlillest/micropython,MrSurly/micropython,PappaPeppar/micropython,warner83/micropython,Peetz0r/micropython-esp32,Timmenem/micropython,noahchense/micropython,mianos/micropython,pozetroninc/micropython,ericsnowcurrently/micropython,tuc-osg/micropython,AriZuu/micropython,drrk/micropython,slzatz/micropython,jmarcelino/pycom-micropython,selste/micropython,neilh10/micropython,praemdonck/micropython,ceramos/micropython,firstval/micropython,xhat/micropython,HenrikSolver/micropython,Vogtinator/micropython,ruffy91/micropython,tralamazza/micropython,misterdanb/micropython,matthewelse/micropython,vitiral/micropython,turbinenreiter/micropython,dxxb/micropython,deshipu/micropython,Timmenem/micropython,deshipu/micropython,alex-robbins/micropython,jmarcelino/pycom-micropython,neilh10/micropython,skybird6672/micropython,turbinenreiter/micropython,xyb/micropython,adafruit/circuitpython,infinnovation/micropython,suda/micropython,mpalomer/micropython,Vogtinator/micropython,jimkmc/micropython,toolmacher/micropython,cwyark/micropython,jmarcelino/pycom-micropython,lowRISC/micropython,kostyll/micropython,EcmaXp/micropython,adafruit/circuitpython,adamkh/micropython,emfcamp/micropython,xyb/micropython,heisewangluo/micropython,mgyenik/micropython,puuu/micropython,cwyark/micropython,KISSMonX/micropython,vriera/micropython,noahwilliamsson/micropython,pramasoul/micropython,utopiaprince/micropython,Timmenem/micropython,torwag/micropython,emfcamp/micropython,toolmacher/micropython,chrisdearman/micropython,henriknelson/micropython,rubencabrera/micropython,MrSurly/micropython-esp32,adafruit/micropython,danicampora/micropython,KISSMonX/micropython,tuc-osg/micropython,infinnovation/micropython,alex-robbins/micropython,suda/micropython,emfcamp/micropython,TDAbboud/micropython,SHA2017-badge/micropython-esp32,ericsnowcurrently/micropython,MrSurly/micropython,jlillest/micropython,swegener/micropython,mhoffma/micropython,adafruit/micropython,galenhz/micropython,KISSMonX/micropython,oopy/micropython,MrSurly/micropython-esp32,emfcamp/micropython,mhoffma/micropython,noahchense/micropython,paul-xxx/micropython,martinribelotta/micropython,orionrobots/micropython,ernesto-g/micropython,slzatz/micropython,ryannathans/micropython,lowRISC/micropython,AriZuu/micropython,hiway/micropython,noahchense/micropython,mgyenik/micropython,ryannathans/micropython,tobbad/micropython,warner83/micropython,ahotam/micropython,aethaniel/micropython,firstval/micropython,jimkmc/micropython,stonegithubs/micropython,ceramos/micropython,ceramos/micropython,cnoviello/micropython,suda/micropython,ericsnowcurrently/micropython,slzatz/micropython,mpalomer/micropython,cwyark/micropython,xuxiaoxin/micropython,ruffy91/micropython,micropython/micropython-esp32,adafruit/circuitpython,henriknelson/micropython,blazewicz/micropython,AriZuu/micropython,ChuckM/micropython,hosaka/micropython,lowRISC/micropython,ernesto-g/micropython,blmorris/micropython,matthewelse/micropython,hosaka/micropython,micropython/micropython-esp32,xhat/micropython,dinau/micropython,jlillest/micropython,jmarcelino/pycom-micropython,neilh10/micropython,firstval/micropython,trezor/micropython,tobbad/micropython,omtinez/micropython,hosaka/micropython,paul-xxx/micropython,ceramos/micropython,henriknelson/micropython,torwag/micropython,danicampora/micropython,swegener/micropython,mgyenik/micropython,cloudformdesign/micropython,adafruit/circuitpython,feilongfl/micropython,skybird6672/micropython,dhylands/micropython,mianos/micropython,blazewicz/micropython,MrSurly/micropython,misterdanb/micropython,dmazzella/micropython,blmorris/micropython,oopy/micropython,Peetz0r/micropython-esp32,xyb/micropython,ericsnowcurrently/micropython,adafruit/circuitpython,infinnovation/micropython,drrk/micropython,tralamazza/micropython,danicampora/micropython,MrSurly/micropython-esp32