image_signing: Add more description to set_gbb_flags.

There are two new GBB flags added (lid/fastboot) and we should update the
description in set_gbb_flags.

BRANCH=none
BUG=none
TEST=emerge-link vboot_reference

Change-Id: I0d16df03e9427ec1c8780fbb6be10c31eed9bf9e
Signed-off-by: Hung-Te Lin <hungte@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/286052
Reviewed-by: Bill Richardson <wfrichar@chromium.org>
1 file changed