oak: change NV_STORAGE from ec to flash

BRANCH=none
BUG=none
TEST=boot to shell and crossystem works after reboot.

Change-Id: I0c8d69fa22d694350c6ef6cbb1b6a724b7649777
Signed-off-by: CC Ma <cc.ma@mediatek.com>
Reviewed-on: https://chromium-review.googlesource.com/299111
Commit-Ready: Yidi Lin <yidi.lin@mediatek.com>
Tested-by: Yidi Lin <yidi.lin@mediatek.com>
Reviewed-by: Julius Werner <jwerner@chromium.org>