Add a constant for the escape key

We use this in a few places, so add a constant.

BUG=chromium:837018
BRANCH=none
TEST=FEATURES=test emerge-grunt --nodeps vboot_reference

Change-Id: I7182d0ac52c23c01397de08683ad83b818486f91
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1286221
Reviewed-by: Randall Spangler <rspangler@chromium.org>
3 files changed