Add a blank screen value into screen index.

A convenient value that firmware to identity the blank screen.

TEST=emerge-tegra2_seaboard vboot_reference-firmware successfully
BUG=chromium-os:1305

Change-Id: Ib38f0c5ba571e95c966ae58c81d22487e416e666

R=clchious@chromium.org

Review URL: http://codereview.chromium.org/6712029
1 file changed