futility: updater: allow changing GBB flags on erased flash

The --gbb_flags supports changing GBB flags but the value will be
ignored if the device flash was erased (e.g., no valid GBB section).
To fix that we should check the 'to' and 'from' flash contents
separately when preserving GBB data.

BUG=b:216295706
TEST=build; emerge test
BRANCH=None

Change-Id: Ie02138dd4234b461ca1913bef1cabde8becf57c9
Signed-off-by: Hung-Te Lin <hungte@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/3414190
Reviewed-by: Yu-Ping Wu <yupingso@chromium.org>
Commit-Queue: Yu-Ping Wu <yupingso@chromium.org>
1 file changed