x86: Update Chromium OS GNVS names

The Global Non-Volatile Storage struct has some fields with particular
meanings. Rename these to make things easier to follow. Also add a few
more boot flags.

GNVS should not be confused with GNVQ (Going Nowhere Very Quickly).

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>