Fix big endian conversion

16-bit quantities are sent big-endian in i2c commands.
Populate buffer with MSB and LSB in a platform independent way.

BUG=chromium-os:21568
TEST=builds clean for x86, arm.
  offset / block_index should be sent big-endian for both arches.
  This should result in a properly backed up image for both arches.

Change-Id: I988c5a050027b2081abd5cb2634cb4221951b6b8
1 file changed