[tls] Restructure to use a single key derivation function master secret

Calls to the key derivation function tls_prf() currently have to pass
the relevant secret (i.e. the pre-master secret or the master secret)
as a parameter.

Restructure to more closely match the design of the TLS version 1.3
key schedule, which maintains a single running secret (which we choose
to name the "key derivation function master secret") that is always
implicitly used as the secret for key expansion.

The secret value is currently used by tls_prf() only as the key to
hmac_init().  We can therefore use hmac_key() to reduce the secret to
a fixed-length value.  (The TLS master secret is already a fixed 48
bytes, but the pre-master secret may be any length.)

The fixed length of the secret is dependent upon the protocol version
and the cipher suite digest algorithm.  For TLS version 1.2, the
length is the HMAC key size for the digest algorithm.  For TLS version
1.1, which uses separate invocations of HMAC-MD5 and HMAC-SHA1, the
length is the sum of the HMAC-MD5 and HMAC-SHA1 key sizes.  (For TLS
version 1.3, the length will be the HKDF key size, i.e. the output
size of the digest algorithm.)

To avoid introducing some very messy memory allocation code paths, we
continue to use a fixed size of 48 bytes for the resumption master
secret stored in the TLS session.  This is sufficient to hold the
48-byte raw master secret for TLS version 1.2 and earlier, and will
also be sufficient to hold the HKDF Derive-Secret output for the
longest supported digest algorithm (SHA-384) in TLS version 1.3.

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