stm32/Makefile: Allow to override CROSS_COMPILE with included Makefile.
stm32/Makefile: Allow to override CROSS_COMPILE with included Makefile.

MrSurly/micropython,pramasoul/micropython,pozetroninc/micropython,tobbad/micropython,pozetroninc/micropython,tobbad/micropython,henriknelson/micropython,pramasoul/micropython,pozetroninc/micropython,selste/micropython,trezor/micropython,trezor/micropython,adafruit/circuitpython,selste/micropython,selste/micropython,tobbad/micropython,MrSurly/micropython,pfalcon/micropython,henriknelson/micropython,bvernoux/micropython,pramasoul/micropython,pfalcon/micropython,trezor/micropython,MrSurly/micropython,selste/micropython,bvernoux/micropython,tobbad/micropython,pfalcon/micropython,bvernoux/micropython,trezor/micropython,tobbad/micropython,MrSurly/micropython,henriknelson/micropython,henriknelson/micropython,kerneltask/micropython,selste/micropython,bvernoux/micropython,pramasoul/micropython,adafruit/circuitpython,MrSurly/micropython,pozetroninc/micropython,adafruit/circuitpython,bvernoux/micropython,adafruit/circuitpython,kerneltask/micropython,kerneltask/micropython,pfalcon/micropython,pramasoul/micropython,adafruit/circuitpython,kerneltask/micropython,henriknelson/micropython,trezor/micropython,pozetroninc/micropython,adafruit/circuitpython,pfalcon/micropython,kerneltask/micropython