The cert_create tool calculates the hash of each BL image and includes
it as an ASN.1 OCTET STRING in the corresponding certificate extension.
Without additional information, the firmware running on the platform
has to know in advance the algorithm used to generate the hash.

This patch modifies the cert_create tool so the certificate extensions
that include an image hash are generated according to the following
ASN.1 structure:

    DigestInfo ::= SEQUENCE {
        digestAlgorithm  AlgorithmIdentifier,
        digest           OCTET STRING
    }

    AlgorithmIdentifier ::=  SEQUENCE  {
        algorithm        OBJECT IDENTIFIER,
        parameters       ANY DEFINED BY algorithm OPTIONAL
    }

The PolarSSL module has been updated to extract the image hash
from the certificate extension according to this structure.

Change-Id: I6d83430f12a8a0eea8447bec7c936e903f644c85