E407/: use the FreeRTOS blinker

Note that there are numerous issues:
- we need to explicitly define USE_STDPERIPH_DRIVER
- there's a compatibility issue with FPU instructions, see
  https://sourceforge.net/projects/freertos/forums/forum/382005/topic/6404907
- builtins trip ld (if not compiling with -static) or create a 500 kB monster
  (with -static)
- the use of vpath is perhaps a bit excessive (we essentially merge all the
  sources into a single virtual directory)

The LED blinks quite erratically. Not sure if that's a bug or a feature.