Add signature verification to keystore and verity signers

This change adds -verify flags to keystore and verity signers for
verifying the signatures of the signed content generated by the
same tools. This allows implementers of verified boot to test and
verify the correctness of their implementations.

Bug: 15984840
Change-Id: I327de9c3a9e035cd11dc5022e978c840cd37581c
5 files changed