invensense: add accelerometer calibration

This commit adds the ability to do accelerometer calibration
in devices using the libinvensense_hal module, such as tuna.

Calibration data is be stored in a 12-byte file /data/accelcalib
as an array of 4-byte floats for X, Y, and Z offsets.
If the file does not exist, default calibration of {0, 0, 0} is
used.

Implementation on tuna:
http://review.cyanogenmod.org/#/c/30956/

NOTE: libsensors will need to be rebuilt for devices where it is
      linked with libinvensense_hal

Change-Id: Ib7f16eb19d46f3fe9d5431529419ea7b6dd1d7d7