ima: add audit log support for larger hashes

Different files might be signed based on different hash algorithms.
This patch prefixes the audit log measurement hash with the hash
algorithm.

Changelog:
- use generic HASH_ALGO defintions
- use ':' as delimiter between the hash algorithm and the digest
  (Roberto Sassu)
- always include the hash algorithm used when audit-logging a measurement

Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
Signed-off-by: Roberto Sassu <roberto.sassu@polito.it>
Signed-off-by: Peter Moody <pmoody@google.com>