signing-script: Update COS signing script to support the new dm format.

Updated the cos signing script to support both old and new version.

BUG=b/240174341
BRANCH=None
TEST=presubmit
RELEASE_NOTE=None

Signed-off-by: Meena Shanmugam <meenashanmugam@google.com>
Change-Id: I8a8747e23ed0dd9cac33824dbd48ab0295864937
Reviewed-on: https://cos-review.googlesource.com/c/third_party/platform/vboot_reference/+/36692
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
Reviewed-by: Robert Kolchmeyer <rkolchmeyer@google.com>