stm32/mboot: Use additional CFLAGS to compile string0.c.
stm32/mboot: Use additional CFLAGS to compile string0.c.

This is the same as a902b69dd51de0e3fe3bb6955296591d6a93abab but applied to
mboot's Makefile.

Signed-off-by: Damien George <99e6b749acbfbe2a596df99e91d24d6e1fdbee00@micropython.org>

MrSurly/micropython,kerneltask/micropython,kerneltask/micropython,pramasoul/micropython,adafruit/circuitpython,henriknelson/micropython,henriknelson/micropython,pramasoul/micropython,henriknelson/micropython,kerneltask/micropython,bvernoux/micropython,henriknelson/micropython,selste/micropython,henriknelson/micropython,MrSurly/micropython,bvernoux/micropython,kerneltask/micropython,selste/micropython,pramasoul/micropython,MrSurly/micropython,adafruit/circuitpython,pramasoul/micropython,adafruit/circuitpython,kerneltask/micropython,selste/micropython,selste/micropython,adafruit/circuitpython,bvernoux/micropython,bvernoux/micropython,adafruit/circuitpython,selste/micropython,MrSurly/micropython,bvernoux/micropython,pramasoul/micropython,MrSurly/micropython,adafruit/circuitpython