Add separate API function stubs for HMAC/CMAC context initialization.

Add separate API function stubs for HMAC/CMAC context initialization.

MACs are symmetric equivalent for asymmetric signatures. For asymmetric
signatures the signing algorithm can be easily deduced from the key. This is
not the case for MAC's where plain symmetric key tells us nothing about the MAC
algorithm we want to use. Considered solutions:

1. Introducing new key types for MACs. Cons:
 - unclear how to handle plain symmetric keys,
 - introduces an artificial division in symmetric keys,
 - new enum values,
 - CMAC cipher has to be set via ctx params.

2. Deducing MAC algorithm from digest algorithm. Digest->HMAC, no digest->CMAC.
   Cons:
 - unclear which algorithm will be used,
 - adding new MAC algorithm may require a significant API change,
 - CMAC cipher has to be set via ctx params.

3. Leaving CMAC as a digest algorithm. Cons:
 - CMAC is not a digest algorithm. It's an equivalent of HMAC, RSA, DSA,...etc,
 - CMAC can't be used for calculating message digest alone,
 - CMAC can't be used as a digest algorithm for HMAC
 - CMAC cipher has to be set via ctx params.

4. Adding new API for CMAC and HMAC context creation. Cons:
 - 1 new functions per MAC algo for context initalization,
 - 1 function for signature comparison
 - low API flexibility

This is an initial commit for solution 4.

Change-Id: I745854fd7b7d87f2c114475b709566ec512d7bbd