Add CTRL-L in dev screen to support a "legacy boot option"

This option is disabled per default and can be enabled with
crossystem dev_boot_legacy=1
or by setting the GBB flag
GBB_FLAG_FORCE_DEV_BOOT_LEGACY      0x00000080

BUG=chrome-os-partner:6108
TEST=crossystem dev_boot_legacy=1
     boot to dev mode screen, press CTRL-L, see SeaBIOS start
     (other CLs needed)
BRANCH=link
Signed-off-by: Stefan Reinauer <reinauer@chromium.org>

Change-Id: I593d2be7cff5ca07b8d08012c4514a172bd75a38
Reviewed-on: https://gerrit.chromium.org/gerrit/31265
Reviewed-by: Stefan Reinauer <reinauer@chromium.org>
Tested-by: Stefan Reinauer <reinauer@chromium.org>
Reviewed-by: Randall Spangler <rspangler@chromium.org>
Commit-Ready: Stefan Reinauer <reinauer@chromium.org>
12 files changed