vboot: Add GBB flag to turn on serial output

Currently this does nothing. This will eventually be used to enable
serial output.

BUG=chromium:210230
BRANCH=none
TEST=none

Change-Id: I5c25fd7406e30b96d12bc4bf8210d3c3f4ae79f1
Signed-off-by: Mary Ruthven <mruthven@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/309716
Reviewed-by: Randall Spangler <rspangler@chromium.org>
3 files changed