zephyr: Automatically derive ARCH.
zephyr: Automatically derive ARCH.

Currently to compile for anything that except ARCH=x86 we have to
provide ARCH via the environment or make arguments. We can do better
than that!

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

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