Introduce config option for experimental TLS 1.3 specific features

Signed-off-by: Hanno Becker <hanno.becker@arm.com>

include/mbedtls/config.h

@@ -1706,6 +1706,22 @@
  */
 #define MBEDTLS_SSL_PROTO_TLS1_2
 
+/**
+ * \def MBEDTLS_SSL_PROTO_TLS1_3_EXPERIMENTAL
+ *
+ * This is an experimental macro used to selectively enable parts
+ * of the code that solely contribute to the ongoing development of
+ * a prototype implementation for TLS 1.3 and DTLS 1.3 but aren't
+ * used otherwise.
+ *
+ * \warning Features under the control of this macro are experimental
+ *          and don't come with any stability guarantees.
+ *
+ * Uncomment this macro to enable experimental and partial
+ * functionality specific to TLS 1.3.
+ */
+#define MBEDTLS_SSL_PROTO_TLS1_3_EXPERIMENTAL
+
 /**
  * \def MBEDTLS_SSL_PROTO_DTLS
  *