mbedtls: add support for pkcs7

PKCS7 signing format is used by OpenPOWER Key Management, which is
using mbedtls as its crypto library.

This patch adds the limited support of pkcs7 parser and verification
to the mbedtls. The limitations are:

* Only signed data is supported.
* CRLs are not currently handled.
* Single signer is supported.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Eric Richter <erichte@linux.ibm.com>
Signed-off-by: Nayna Jain <nayna@linux.ibm.com>

scripts/config.py

@@ -306,6 +306,7 @@ def include_in_crypto(name):
     if name in [
             'MBEDTLS_DEBUG_C', # part of libmbedtls
             'MBEDTLS_NET_C', # part of libmbedtls
+            'MBEDTLS_PKCS7_C', # part of libmbedx509
     ]:
         return False
     return True