Fork vboot_reference in manifest

We need to fork vboot_reference to carry custom patches for signing
behavior.

BUG=b/170128847
TEST=repo init and repo sync
RELEASE_NOTE=None

Change-Id: I7afe7f22b9426364c931cd0f76eb4918849a7fd6
Reviewed-on: https://cos-review.googlesource.com/c/cos/manifest/+/15493
Reviewed-by: Robert Kolchmeyer <rkolchmeyer@google.com>
Tested-by: Dexter Rivera <riverade@google.com>
Reviewed-on: https://cos-review.googlesource.com/c/cos/manifest/+/21891