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/library/ssl_misc.h b/library/ssl_misc.h
index ed3c4a7..9f7ab7f 100644
--- a/library/ssl_misc.h
+++ b/library/ssl_misc.h
@@ -279,7 +279,7 @@
 
 /* This macro determines whether a ciphersuite using a
  * stream cipher can be used. */
-#if defined(MBEDTLS_CIPHER_NULL_CIPHER)
+#if defined(MBEDTLS_SSL_NULL_CIPHERSUITES)
 #define MBEDTLS_SSL_SOME_SUITES_USE_STREAM
 #endif