USB Support

[simoninns]

Given that the new 7_6 PCB revision has support for USB it would be nice to support USB memory sticks for simple FAT file transfer.

[simoninns]

Latest commit adds a usb.c and usb.h for the USB subsystem to be added. There is now code which checks the 'USB hardware capable' resistor added to board revision 7_7 and the debug will indicate if the physical board has the capability to support USB (7_7 and above) or not (7_6 and below).