MIDIUSB flag in boards.h

Added test of MIDIUSB flag to boards.h in order to select between
BOARD_HAVE_SERIALUSB and BOARD_HAVE_MIDIUSB and control the USB
device that gets built.
Renamed midi_usb.c to midi_usb_device
Renamed usb_midi to usb_midi_device
Minor cleanup of the MIDI USB stuff