In order to for tests to pass from the previous commit (which it mandatory for all pk verify/sign
functions to be given a hash_len that is exactly equal to the message digest length of md_alg) the
hash_len that is supplied to the fucntion cannot be MBEDTLS_MD_MAX_SIZE. This would result in all tests failing. Since the md alg for all of these funtions are SHA256, we can use mbedtls functions to get
the required length of a SHA256 digest (32 bytes). Then that number can be used for allocating the
hash buffer.
Signed-off-by: Nick Child <nick.child@ibm.com>