[chibios] add chibios-libopencm3 support

Only the realtime scheduler and SDIO driver from ChibiOS is used.
This brings the support of the SD log on Apogee boards. The current
solution is not fully satisfactory and is an intermediate solution
before switching to a 'pure' ChibiOS arch.
Some of the implementations can be cleaned and/or improved, but since it
is not a long term solution, I doubt it is worth the effort here.
Normal bare-metal libopencm3 code is of course still working.
Git submodules:
- chibios: 2.6.2
- fatfs: patched version for chibios+pprz