Update the open HAL code to Perforce rev 1988459.

There's a few things of note here:

* A new chainmask option for managing configured versus calibrated
  versus transmitting TX chains.  One of the recent board designs
  uses two chains on a three-chain chip and there's some complexity
  behind getting everything exactly right.

* Extend the beacon interval API to include a "fraction" field.
  This is to offset some losses due to dividing, TSF, TU and TU/8
  conversion.

* There's a keycache issue that needs to be better understood, but
  apparently there's some occasional keycache corruption.  The new
  keycache routines allow for reading the keycache entry back
  and comparing it against the existing copy.

* Various initval, calibration fixes and other miscellaneous stuff
  that creeps in from time to time.