[tls] Allow for NIST elliptic curve point formats

[tls] Allow for NIST elliptic curve point formats

The elliptic curve point representation for the x25519 curve includes
only the X value, since the curve is designed such that the Montgomery
ladder does not need to ever know or calculate a Y value.  There is no
curve point format byte: the public key data is simply the X value.
The pre-master secret is also simply the X value of the shared secret
curve point.

The point representation for the NIST curves includes both X and Y
values, and a single curve point format byte that must indicate that
the format is uncompressed.  The pre-master secret for the NIST curves
does not include both X and Y values: only the X value is used.

Extend the definition of an elliptic curve to allow the point size to
be specified separately from the key size, and extend the definition
of a TLS named curve to include an optional curve point format byte
and a pre-master secret length.

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