mpy-cross/Makefile: Guard "override undefine" by test for make feature.
mpy-cross/Makefile: Guard "override undefine" by test for make feature.

make v3.81 doesn't have "undefine" so we can't use it with that version.

pfalcon/micropython,dmazzella/micropython,henriknelson/micropython,pozetroninc/micropython,tobbad/micropython,MrSurly/micropython-esp32,deshipu/micropython,dmazzella/micropython,henriknelson/micropython,tobbad/micropython,adafruit/circuitpython,trezor/micropython,bvernoux/micropython,swegener/micropython,TDAbboud/micropython,adafruit/circuitpython,MrSurly/micropython-esp32,Timmenem/micropython,blazewicz/micropython,torwag/micropython,bvernoux/micropython,trezor/micropython,pramasoul/micropython,tralamazza/micropython,torwag/micropython,PappaPeppar/micropython,kerneltask/micropython,hiway/micropython,deshipu/micropython,micropython/micropython-esp32,MrSurly/micropython-esp32,pozetroninc/micropython,trezor/micropython,blazewicz/micropython,oopy/micropython,bvernoux/micropython,infinnovation/micropython,SHA2017-badge/micropython-esp32,pozetroninc/micropython,swegener/micropython,alex-robbins/micropython,bvernoux/micropython,tralamazza/micropython,AriZuu/micropython,TDAbboud/micropython,henriknelson/micropython,pramasoul/micropython,TDAbboud/micropython,adafruit/micropython,Timmenem/micropython,selste/micropython,oopy/micropython,SHA2017-badge/micropython-esp32,MrSurly/micropython,cwyark/micropython,PappaPeppar/micropython,chrisdearman/micropython,kerneltask/micropython,adafruit/circuitpython,AriZuu/micropython,AriZuu/micropython,infinnovation/micropython,pfalcon/micropython,MrSurly/micropython,tobbad/micropython,chrisdearman/micropython,deshipu/micropython,HenrikSolver/micropython,HenrikSolver/micropython,infinnovation/micropython,PappaPeppar/micropython,swegener/micropython,trezor/micropython,henriknelson/micropython,selste/micropython,adafruit/micropython,adafruit/micropython,chrisdearman/micropython,adafruit/micropython,oopy/micropython,cwyark/micropython,oopy/micropython,hiway/micropython,pozetroninc/micropython,TDAbboud/micropython,Timmenem/micropython,HenrikSolver/micropython,blazewicz/micropython,ryannathans/micropython,kerneltask/micropython,pfalcon/micropython,cwyark/micropython,SHA2017-badge/micropython-esp32,Timmenem/micropython,dmazzella/micropython,blazewicz/micropython,tobbad/micropython,henriknelson/micropython,tobbad/micropython,adafruit/circuitpython,tralamazza/micropython,trezor/micropython,pfalcon/micropython,adafruit/circuitpython,Timmenem/micropython,tralamazza/micropython,MrSurly/micropython-esp32,hiway/micropython,alex-robbins/micropython,infinnovation/micropython,selste/micropython,torwag/micropython,AriZuu/micropython,swegener/micropython,HenrikSolver/micropython,PappaPeppar/micropython,TDAbboud/micropython,bvernoux/micropython,SHA2017-badge/micropython-esp32,infinnovation/micropython,dmazzella/micropython,lowRISC/micropython,pramasoul/micropython,lowRISC/micropython,ryannathans/micropython,chrisdearman/micropython,kerneltask/micropython,torwag/micropython,deshipu/micropython,MrSurly/micropython-esp32,pfalcon/micropython,pozetroninc/micropython,MrSurly/micropython,torwag/micropython,lowRISC/micropython,alex-robbins/micropython,adafruit/micropython,blazewicz/micropython,micropython/micropython-esp32,hiway/micropython,ryannathans/micropython,pramasoul/micropython,selste/micropython,alex-robbins/micropython,pramasoul/micropython,SHA2017-badge/micropython-esp32,ryannathans/micropython,chrisdearman/micropython,cwyark/micropython,lowRISC/micropython,AriZuu/micropython,MrSurly/micropython,lowRISC/micropython,deshipu/micropython,swegener/micropython,HenrikSolver/micropython,kerneltask/micropython,MrSurly/micropython,cwyark/micropython,hiway/micropython,oopy/micropython,micropython/micropython-esp32,ryannathans/micropython,selste/micropython,adafruit/circuitpython,micropython/micropython-esp32,PappaPeppar/micropython,micropython/micropython-esp32,alex-robbins/micropython