The unique serial number from the device unique signature block was never used
in any examples. Add it to the these two usb midi examples, to have a user for
this api available for reference.
Be specifically careful with the usb examples. There is likely some
breakage to be expected, not sure I updated all the drivers to the
correct types for the respective chips.
Tested on STM32F4Discovery with Linux host using both amidi (raw
messages to/from device) and a synthesizer (note on/off) and on
Windows 7 using MIDI-OX.
