[tls] Abstract out concept of a TLS authentication header

[tls] Abstract out concept of a TLS authentication header

All TLS cipher types use a common structure for the per-record data
that is authenticated in addition to the plaintext itself.  This data
is used as a prefix in the HMAC calculation for stream and block
ciphers, or as additional authenticated data for AEAD ciphers.

Define a "TLS authentication header" structure to hold this data as a
contiguous block, in order to meet the alignment requirement for AEAD
ciphers such as GCM.

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