stm32/mboot/Makefile: Define "BUILDING_MBOOT" when building mboot.
stm32/mboot/Makefile: Define "BUILDING_MBOOT" when building mboot.

So boards can configure their settings based on whether mboot or the main
firmware is being built.

pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython