futility: Wipe GBB data before writing new values.

The previous version of gbb_utility always zeros data before writing new values
and we should keep this behavior, to simplify firmware hash calculation and
potential security concern.

BRANCH=none
BUG=chromium:413066
TEST=emerge gbb_utility; factory/bin/gooftool get_firmware_hash bios.bin
Originial-Change-Id: Ic97a118cefc9698d52d9370b627670ff103d5e23

Change-Id: If199040909c4719c89dbc52fd7488d69abd981e5
Reviewed-on: https://chromium-review.googlesource.com/217478
Reviewed-by: Hung-Te Lin <hungte@chromium.org>
Commit-Queue: Hung-Te Lin <hungte@chromium.org>
Tested-by: Hung-Te Lin <hungte@chromium.org>
1 file changed