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>
diff --git a/tests/suites/test_suite_ssl_decrypt.function b/tests/suites/test_suite_ssl_decrypt.function
index 37265de..7a22939 100644
--- a/tests/suites/test_suite_ssl_decrypt.function
+++ b/tests/suites/test_suite_ssl_decrypt.function
@@ -13,7 +13,7 @@
  * END_DEPENDENCIES
  */
 
-/* BEGIN_CASE depends_on:MBEDTLS_SSL_PROTO_TLS1_2:MBEDTLS_CIPHER_NULL_CIPHER */
+/* BEGIN_CASE depends_on:MBEDTLS_SSL_PROTO_TLS1_2:MBEDTLS_SSL_NULL_CIPHERSUITES */
 void ssl_decrypt_null(int hash_id)
 {
     mbedtls_ssl_transform transform_in, transform_out;