Platform support for the ATMEL AT91SAM7[XS] ARM7 microcontrollers. This is a base platform -- it needs to be specialized for a particular board (see sam7ex256 as an example of this) to be useful. In particular it does not provide the platform/board.h (which must include the correct at91sam7*h file and mux config). TODO: - support clock rates other than (and above) 18MHz - compute uart divisor, PIT interval, etc from MCK - timer hook should honor the requested interval - current_time() should return a meaningful value - actually pass argument to interrupt handlers?