appended signatures: Parse PKCS#7 signed data

appended signatures: Parse PKCS#7 signed data

This code allows us to parse:

 - PKCS#7 signed data messages. Only a single signer info is supported, which
   is all that the Linux sign-file utility supports creating out-of-the-box.
   Only RSA, SHA-256 and SHA-512 are supported. Any certificate embedded in
   the PKCS#7 message will be ignored.

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>