appended signatures: Support verifying appended signatures

appended signatures: Support verifying appended signatures

Building on the parsers and the ability to embed X.509 certificates, as well
as the existing gcrypt functionality, add a module for verifying appended
signatures.

This includes a signature verifier that requires that the Linux kernel and
GRUB modules have appended signatures for verification.

Signature verification must be enabled by setting check_appended_signatures.
If secure boot is enabled with enforce mode when the appendedsig module is
loaded, signature verification will be enabled, and trusted keys will be
extracted from the GRUB ELF Note and stored in the db and locked automatically.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Sudhakar Kuppusamy <sudhakar@linux.ibm.com>
Reviewed-by: Avnish Chouhan <avnish@linux.ibm.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>