stm32/main: Guard usb_mode lines in default boot.py by relevant #if.
stm32/main: Guard usb_mode lines in default boot.py by relevant #if.

selste/micropython,adafruit/circuitpython,MrSurly/micropython,kerneltask/micropython,henriknelson/micropython,kerneltask/micropython,kerneltask/micropython,ryannathans/micropython,pramasoul/micropython,adafruit/circuitpython,tralamazza/micropython,pfalcon/micropython,pozetroninc/micropython,dmazzella/micropython,dmazzella/micropython,MrSurly/micropython,tobbad/micropython,pramasoul/micropython,pozetroninc/micropython,adafruit/circuitpython,tobbad/micropython,henriknelson/micropython,ryannathans/micropython,swegener/micropython,tobbad/micropython,pfalcon/micropython,tralamazza/micropython,swegener/micropython,adafruit/circuitpython,pramasoul/micropython,henriknelson/micropython,adafruit/circuitpython,ryannathans/micropython,trezor/micropython,pfalcon/micropython,dmazzella/micropython,trezor/micropython,pramasoul/micropython,trezor/micropython,pozetroninc/micropython,kerneltask/micropython,swegener/micropython,bvernoux/micropython,pozetroninc/micropython,pozetroninc/micropython,MrSurly/micropython,MrSurly/micropython,swegener/micropython,trezor/micropython,pfalcon/micropython,ryannathans/micropython,adafruit/micropython,selste/micropython,trezor/micropython,pramasoul/micropython,bvernoux/micropython,bvernoux/micropython,kerneltask/micropython,bvernoux/micropython,pfalcon/micropython,ryannathans/micropython,henriknelson/micropython,tralamazza/micropython,tralamazza/micropython,MrSurly/micropython,adafruit/micropython,tobbad/micropython,selste/micropython,adafruit/micropython,selste/micropython,adafruit/circuitpython,bvernoux/micropython,selste/micropython,adafruit/micropython,henriknelson/micropython,swegener/micropython,dmazzella/micropython,adafruit/micropython,tobbad/micropython