stm32f4: usb_midi: Introduce example MIDI firmware

Tested on STM32F4Discovery with Linux host using both amidi (raw
messages to/from device) and a synthesizer (note on/off).

This example depends upon changes to libopencm3 and should not be merged until the libopencm3 submodule includes the following:
https://github.com/libopencm3/libopencm3/pull/309
we'll _probably_ want to put something more generic here, I think?