[tls] Formalise notions of fixed and record initialisation vectors

[tls] Formalise notions of fixed and record initialisation vectors

TLS block ciphers always use CBC (as per RFC 5246 section 6.2.3.2)
with a record initialisation vector length that is equal to the cipher
block size, and no fixed initialisation vector.

The initialisation vector for AEAD ciphers such as GCM is less
straightforward, and requires both a fixed and per-record component.

Extend the definition of a cipher suite to include fixed and record
initialisation vector lengths, and generate the fixed portion (if any)
as part of key expansion.

Do not add explicit calls to cipher_setiv() in tls_assemble_block()
and tls_split_block(), since the constraints imposed by RFC 5246 are
specifically chosen to allow implementations to avoid doing so.
(Instead, add a sanity check that the record initialisation vector
length is equal to the cipher block size.)

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