[crypto] Define a structure for holding hybrid MD5+SHA1 HMAC keys

The hybrid PRF used in TLS version 1.1 and earlier does not use HMAC
with the hybrid MD5+SHA1 algorithm: it uses separate invocations of
HMAC-MD5 and HMAC-SHA1.

Using hmac_keysize(&md5_sha1_algorithm) would produce a size too small
to hold the combined HMAC-MD5 and HMAC-SHA1 keys.  One option would be
to set the (currently unused) MD5+SHA1 block size to 128, thereby
ensuring that hmac_keysize() would happen to return a length large
enough to hold both HMAC keys.  This would avoid the need to
special-case the MD5+SHA1 algorithm when calculating the required HMAC
key size for the PRF, but would inevitably cause confusion in future.

Set the MD5+SHA1 block size to 64 (since both algorithms have the same
underlying block size, and this would therefore produce the "correct"
result if anything were ever to use HMAC directly with the hybrid
MD5+SHA1 algorithm), and define a separate structure for holding the
separated HMAC keys used by the PRF in TLS version 1.1 and earlier.

Signed-off-by: Michael Brown <mcb30@ipxe.org>