Add support for the Texas Instruments CC2538 ARM Cortex-M3 MCU and developer kit.

I would prefer you 'hard-code' the LED initialization (and also the LED control macros in `board.h`). The reason is, that the LED macros are often used for debugging. Having minimal overhead control enables developers to use the LEDs also for measuring timings (as i.e. interrupt latency). This is why we implemented the LED control stuff so far directly on top of the registers for all existing boards/cpus...