E407/: build "imported" object files at their origin
E407/: use += to add to Makefile variables (instead of renaming)

This is simpler and is we can probably tolerate Makefile.stm32 changing
CFLAGS and SRC behind our back.
E407/: generate FreeRTOSConfig.h; make "clean" and "spotless" extendable

We just trust that the good folks from CORTEX_M4F_STM32F407ZG-SK
chose their config variables wisely.
E407/Makefile.stm32: don't cherry-pick files from the demo builder

Just build them in place, like we do for everything else now.
E407/: properly handle demo builder dependencies in project-specific Makefile