chromeos-bootimage: Use tianocore as the default for legacy

With altfw we currently don't define a default bootloader. For now let's
use Tianocore for that, if it is available.

BUG=chromium:837018
BRANCH=grunt
TEST=FW_NAME=grunt USE="altfw u-boot tianocore -cb_legacy_tianocore em100-mode" \
	emerge-grunt --nodeps -q chromeos-bootimage
See that '0' shows up as an option, and boots TianoCore

Change-Id: I7b300d81ebb6d52b6e3a65c0921762d0681b2559
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1274165
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Reviewed-by: Julius Werner <jwerner@chromium.org>
2 files changed