platforms/exp5438_25*: clean up ActiveMessage.c (cosmetic)
cc2520-v2: remove CCA, FIFO, and FIFOP no longer used.
cc2520-v2: remove platform pins from driver.  Move to platform/Hpl.

- tos/platforms/<...>/hardware/cc2520/Hpl... owns the pins.
- wire from Hpl to PlatformCC2520.
- remove Platform pin references in cc2520-v2/CC2520DriverLayerC (wrong place)
- finish removing CCA, FIFO, and FIFOP
cc2520-v2: add guts to 2591 implementation

- CC2520_2591 define to control a few values that change when the 2591 is
  in the loop.
- update various register values to reflect the TIMAC code.
- AGCCTRL1 is now a platform dependent value.  The presence of the 2591
  tweaks what it needs to be set to.
- add guts to setHighGain and setLowGain
cc2520-v2: implement guts of RadioCCA.request (checkCCA)
cc2520-v2: move XOSC powerdown into PlatformCC2520.sleep
cc2520-v2: update documentation to reflect the 2591