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) and on
Windows 7 using MIDI-OX.

examples/stm32/f4/stm32f4-discovery/usb_midi/README

@@ -0,0 +1,49 @@
+------------------------------------------------------------------------------
+README
+------------------------------------------------------------------------------
+
+This example implements a USB MIDI device to demonstrate the use of the
+USB device stack. It implements the device configuration found in Appendix
+B of the Universal Serial Bus Device Class Definition for MIDI Devices.
+
+The 'USER' button sends note on/note off messages.
+
+The board will also react to identity request (or any other data sent to
+the board) by transmitting an identity message in reply.
+
+Board connections:
+------------------
+
+  CN5	(USB_OTG_FS)	USB acting as device, connect to computer
+
+Testing
+-------
+
+To list midi devices, which should include this demo device
+
+    $ amidi -l
+    Dir Device    Name
+    IO  hw:2,0,0  MIDI demo MIDI 1
+    $
+
+To record events, while pushing the user button
+
+    $ amidi -d -p hw:2,0,0
+    
+    90 3C 40   -- key down
+    80 3C 40   -- key up
+    90 3C 40
+    80 3C 40^C
+    12 bytes read
+    $
+
+To query the system identity, note this dump matches sysex_identity[] in the
+source.
+
+    $ amidi -d -p hw:2,0,0 -s Sysexdump.syx
+    
+    F0 7E 00 7D 66 66 51 19 00 00 01 00 F7
+    ^C
+    13 bytes read
+    $
+