[tls] Add support for the Extended Master Secret

RFC 7627 defines the Extended Master Secret (EMS) as an alternative
calculation that uses the digest of all handshake messages rather than
just the client and server random bytes.

Add support for negotiating the Extended Master Secret extension and
performing the relevant calculation of the master secret.

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

src/include/ipxe/tls.h

@@ -134,6 +134,9 @@ struct tls_header {
 /* TLS signature algorithms extension */
 #define TLS_SIGNATURE_ALGORITHMS 13
 
+/* TLS extended master secret extension */
+#define TLS_EXTENDED_MASTER_SECRET 23
+
 /* TLS session ticket extension */
 #define TLS_SESSION_TICKET 35
 
@@ -452,6 +455,8 @@ struct tls_connection {
 	uint8_t *handshake_ctx;
 	/** Secure renegotiation flag */
 	int secure_renegotiation;
+	/** Extended master secret flag */
+	int extended_master_secret;
 	/** Verification data */
 	struct tls_verify_data verify;