
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?