image_signing: support signing of OCI containers

BUG=chromium:660209
TEST=`./sign_official_build.sh oci-container fastboot/ ../tests/devkeys` works
TEST=signing an image inserted the container pubkey
BRANCH=None

Change-Id: I75793b03e93f2c18b1495a3ec729ad04d2e17401
Reviewed-on: https://chromium-review.googlesource.com/427538
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: David Riley <davidriley@chromium.org>
5 files changed