firmware: Add developer menu support for detachables

Adding in basic menu support (using arrow keys)
for detachables, which do not have keyboards.  Thus,
we can only do menu scrolling rather than the old
ctrl+D/U/L way.  This is specifically for the developer
warning menu.

BUG=chrome-os-partner:61275
BRANCH=None
TEST=reboot system and try scrolling through menu

Change-Id: I39f56f5ca2fedd47df53ab246ec357d07c9c4af0
Signed-off-by: Shelley Chen <shchen@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/424353
Reviewed-by: Randall Spangler <rspangler@chromium.org>
3 files changed