appended signatures: GRUB commands to manage the certificates

appended signatures: GRUB commands to manage the certificates

Introducing the following GRUB commands to manage the certificates.

 1. append_list_db:
      Show the list of trusted certificates from the db list
 2. append_add_db_cert:
      Add the trusted certificate to the db list
 3. append_add_dbx_cert:
      Add the distrusted certificate to the dbx list
 4. append_verify:
      Verify the signed file using db list

Note that if signature verification (check_appended_signatures) is set to yes,
the append_add_db_cert and append_add_dbx_cert commands only accept the file
‘X509_certificate’ that is signed with an appended signature.

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