platform/exp5438_2520: fix Msp430TimerMicroMap.

formerly, the map had T0A1 allocated for the SFDCapture.  But this moved
to T0A3 (in HplCC2520C).  But the MicroMap didn't change to reflect.
Fix it.
platform/exp5438_2591: PlatformPins shouldn't touch gp5

The dual chip 2520/2591 implementation has gp5 wired directly and the
MCU doesn't have it.  So don't initialize the non-existent pin.
platforms/exp5438_25{20,91}: clean up, remove various kruft
cc2520-v2: more clean up
cc2520-v2: add a window where pending interrupts can happen

Every iteration of processExceptions, we let pending interrupts occur
before actually starting to accessing the cc2520 chip.  This makes
this driver play better with other interrupt users.

We still also let interrupts in while spending time pulling bytes
from the RXFIFO (snarfMsg).  This hasn't changed.