stm32/usb: Allow board to select which USBD is used as the main one.
stm32/usb: Allow board to select which USBD is used as the main one.

By defining MICROPY_HW_USB_MAIN_DEV a given board can select to use either
USB_PHY_FS_ID or USB_PHY_HS_ID as the main USBD peripheral, on which the
REPL will appear.  If not defined this will be automatically configured.

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