Introduce MBEDTLS_SSL_NULL_CIPHERSUITES

The support for TLS ciphersuites without
encryption does not rely anymore on the
MBEDTLS_CIPHER_NULL_CIPHER feature of
the cipher module. Introduce a specific
config option to enable these ciphersuites
and use it instead of MBEDTLS_CIPHER_NULL_CIPHER.

Signed-off-by: Ronald Cron <ronald.cron@arm.com>

include/mbedtls/mbedtls_config.h

@@ -191,6 +191,18 @@
  * \{
  */
 
+/**
+ * \def MBEDTLS_SSL_NULL_CIPHERSUITES
+ *
+ * Enable ciphersuites without encryption.
+ *
+ * Warning: Only do so when you know what you are doing. This allows for
+ * channels without any encryption. All data are transmitted in clear.
+ *
+ * Uncomment this macro to enable the NULL ciphersuites
+ */
+//#define MBEDTLS_SSL_NULL_CIPHERSUITES
+
 /**
  * \def MBEDTLS_DEBUG_C
  *