stm32/mboot: Set USE_MBOOT=1 by default in the Makefile.
stm32/mboot: Set USE_MBOOT=1 by default in the Makefile.

This allows boards that need USE_MBOOT to be built properly whether or not
USE_MBOOT=1 is specified when building mboot.

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