[tls] Add MAC length as a cipher suite parameter

[tls] Add MAC length as a cipher suite parameter

TLS stream and block ciphers use a MAC with a length equal to the
output length of the digest algorithm in use.  For AEAD ciphers there
is no MAC, with the equivalent functionality provided by the cipher
algorithm's authentication tag.

Allow for the existence of AEAD cipher suites by making the MAC length
a parameter of the cipher suite.

Assume that the MAC key length is equal to the MAC length, since this
is true for all currently supported cipher suites.

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