crypto_config: Migrated MBEDTLS_NULL_CIPHER in legacy-crypto

Signed-off-by: Minos Galanakis <minos.galanakis@arm.com>
diff --git a/docs/proposed/config-split.md b/docs/proposed/config-split.md
index b91d55e..9f8d734 100644
--- a/docs/proposed/config-split.md
+++ b/docs/proposed/config-split.md
@@ -311,6 +311,7 @@
 #define MBEDTLS_CIPHER_MODE_CTR
 #define MBEDTLS_CIPHER_MODE_OFB
 #define MBEDTLS_CIPHER_MODE_XTS
+//#define MBEDTLS_CIPHER_NULL_CIPHER
 #define MBEDTLS_CIPHER_PADDING_ONE_AND_ZEROS
 #define MBEDTLS_CIPHER_PADDING_PKCS7
 #define MBEDTLS_CIPHER_PADDING_ZEROS
@@ -382,7 +383,6 @@
 
 #### SECTION TLS feature selection
 ```
-//#define MBEDTLS_CIPHER_NULL_CIPHER
 #define MBEDTLS_DEBUG_C
 #define MBEDTLS_KEY_EXCHANGE_DHE_PSK_ENABLED
 #define MBEDTLS_KEY_EXCHANGE_DHE_RSA_ENABLED