[tls] Refuse to resume sessions with mismatched master secret methods

RFC 7627 section 5.3 states that the client must abort the handshake
if the server attempts to resume a session where the master secret
calculation method stored in the session does not match the method
used for the connection being resumed.

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

src/include/ipxe/tls.h

@@ -353,6 +353,8 @@ struct tls_session {
 	size_t ticket_len;
 	/** Master secret */
 	uint8_t master_secret[48];
+	/** Extended master secret flag */
+	int extended_master_secret;
 
 	/** List of connections */
 	struct list_head conn;