image_signing: use GSC nomenclature instead of Cr50

This patch does not yet provide the ability to sign Ti50 images, but
prepares the signing scripts for further modifications to support a
variety of security chip signing flows.

BRANCH=none
BUG=b:173049030
TEST=verified successful signing of a Cr50 image in a test signer
     setup
     also created a functional Cr50 image invoking
     sign_official_build.sh by hand.

Change-Id: Ic103c9fdf7d1c4ea160c7f6849d5ae5a8303c343
Signed-off-by: Vadim Bendebury <vbendeb@google.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/2537078
Tested-by: Vadim Bendebury <vbendeb@chromium.org>
Reviewed-by: George Engelbrecht <engeg@google.com>
Commit-Queue: Vadim Bendebury <vbendeb@chromium.org>
Auto-Submit: Vadim Bendebury <vbendeb@chromium.org>
2 files changed