zephyr: Use recently added "make outputexports" Zephyr target.
zephyr: Use recently added "make outputexports" Zephyr target.

The outputexpors target, which exports Zephyr environment variables, was
recently added to Zephyr. By exploiting this feature we can hugely simplify
the build system, improving robustness at the same time.

Signed-off-by: Daniel Thompson <4421c302a8f2019ff02452a16ddbdd37770f814c@linaro.org>

TDAbboud/micropython,hosaka/micropython,HenrikSolver/micropython,bvernoux/micropython,bvernoux/micropython,trezor/micropython,AriZuu/micropython,Peetz0r/micropython-esp32,tralamazza/micropython,pfalcon/micropython,puuu/micropython,ryannathans/micropython,pramasoul/micropython,torwag/micropython,henriknelson/micropython,PappaPeppar/micropython,matthewelse/micropython,swegener/micropython,toolmacher/micropython,toolmacher/micropython,alex-march/micropython,deshipu/micropython,mhoffma/micropython,tobbad/micropython,dxxb/micropython,matthewelse/micropython,SHA2017-badge/micropython-esp32,SHA2017-badge/micropython-esp32,tralamazza/micropython,cwyark/micropython,turbinenreiter/micropython,hiway/micropython,pozetroninc/micropython,hiway/micropython,hosaka/micropython,SHA2017-badge/micropython-esp32,MrSurly/micropython,tuc-osg/micropython,tuc-osg/micropython,pozetroninc/micropython,alex-march/micropython,cwyark/micropython,matthewelse/micropython,MrSurly/micropython,hosaka/micropython,cwyark/micropython,puuu/micropython,Peetz0r/micropython-esp32,micropython/micropython-esp32,adafruit/micropython,pramasoul/micropython,Peetz0r/micropython-esp32,bvernoux/micropython,selste/micropython,HenrikSolver/micropython,Timmenem/micropython,tobbad/micropython,Timmenem/micropython,oopy/micropython,MrSurly/micropython-esp32,blazewicz/micropython,PappaPeppar/micropython,jmarcelino/pycom-micropython,selste/micropython,selste/micropython,mhoffma/micropython,selste/micropython,trezor/micropython,kerneltask/micropython,oopy/micropython,alex-robbins/micropython,MrSurly/micropython-esp32,torwag/micropython,blazewicz/micropython,adafruit/micropython,alex-robbins/micropython,pramasoul/micropython,pozetroninc/micropython,chrisdearman/micropython,ryannathans/micropython,pfalcon/micropython,AriZuu/micropython,pozetroninc/micropython,jmarcelino/pycom-micropython,ryannathans/micropython,adafruit/circuitpython,tobbad/micropython,TDAbboud/micropython,tralamazza/micropython,puuu/micropython,adafruit/circuitpython,turbinenreiter/micropython,cwyark/micropython,MrSurly/micropython,adafruit/micropython,kerneltask/micropython,adafruit/micropython,jmarcelino/pycom-micropython,bvernoux/micropython,puuu/micropython,Timmenem/micropython,dmazzella/micropython,oopy/micropython,SHA2017-badge/micropython-esp32,swegener/micropython,chrisdearman/micropython,Timmenem/micropython,matthewelse/micropython,MrSurly/micropython,jmarcelino/pycom-micropython,jmarcelino/pycom-micropython,MrSurly/micropython-esp32,dmazzella/micropython,MrSurly/micropython-esp32,trezor/micropython,lowRISC/micropython,Timmenem/micropython,MrSurly/micropython-esp32,hiway/micropython,adafruit/micropython,tralamazza/micropython,tobbad/micropython,deshipu/micropython,micropython/micropython-esp32,HenrikSolver/micropython,Peetz0r/micropython-esp32,lowRISC/micropython,dxxb/micropython,pfalcon/micropython,toolmacher/micropython,tuc-osg/micropython,dxxb/micropython,ryannathans/micropython,HenrikSolver/micropython,swegener/micropython,infinnovation/micropython,alex-march/micropython,mhoffma/micropython,cwyark/micropython,dxxb/micropython,torwag/micropython,swegener/micropython,TDAbboud/micropython,hosaka/micropython,adafruit/circuitpython,hosaka/micropython,blazewicz/micropython,micropython/micropython-esp32,PappaPeppar/micropython,infinnovation/micropython,micropython/micropython-esp32,dmazzella/micropython,tuc-osg/micropython,turbinenreiter/micropython,lowRISC/micropython,infinnovation/micropython,deshipu/micropython,deshipu/micropython,pfalcon/micropython,alex-robbins/micropython,SHA2017-badge/micropython-esp32,micropython/micropython-esp32,mhoffma/micropython,TDAbboud/micropython,turbinenreiter/micropython,mhoffma/micropython,chrisdearman/micropython,dmazzella/micropython,adafruit/circuitpython,toolmacher/micropython,alex-robbins/micropython,AriZuu/micropython,oopy/micropython,torwag/micropython,hiway/micropython,bvernoux/micropython,ryannathans/micropython,infinnovation/micropython,Peetz0r/micropython-esp32,deshipu/micropython,pramasoul/micropython,AriZuu/micropython,matthewelse/micropython,swegener/micropython,tobbad/micropython,adafruit/circuitpython,toolmacher/micropython,HenrikSolver/micropython,MrSurly/micropython,henriknelson/micropython,turbinenreiter/micropython,pramasoul/micropython,lowRISC/micropython,kerneltask/micropython,chrisdearman/micropython,trezor/micropython,henriknelson/micropython,AriZuu/micropython,puuu/micropython,henriknelson/micropython,henriknelson/micropython,oopy/micropython,PappaPeppar/micropython,blazewicz/micropython,infinnovation/micropython,blazewicz/micropython,alex-march/micropython,hiway/micropython,lowRISC/micropython,trezor/micropython,torwag/micropython,alex-robbins/micropython,pozetroninc/micropython,PappaPeppar/micropython,selste/micropython,chrisdearman/micropython,tuc-osg/micropython,pfalcon/micropython,kerneltask/micropython,kerneltask/micropython,adafruit/circuitpython,matthewelse/micropython,dxxb/micropython,alex-march/micropython,TDAbboud/micropython