certs: dm-verity: add keyring certification for verity

Enable SYSTEM_TRUSTED_KEYS and add certification
for x509 signing for system.

CRs-Fixed: 1116507
Change-Id: I01298d75da0ff9faee2d01118107d53e9a2aef8a
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>
Signed-off-by: Rishabh Bhatnagar <rishabhb@codeaurora.org>