Add mki value and some review comments

1. Add check for prerequisites in check_config.h
2. Add mki value to use_srtp extension
3. address some review comments

Signed-off-by: Johan Pascal <johan.pascal@belledonne-communications.com>
diff --git a/include/mbedtls/check_config.h b/include/mbedtls/check_config.h
index 120c1d3..fd979db 100644
--- a/include/mbedtls/check_config.h
+++ b/include/mbedtls/check_config.h
@@ -871,6 +871,10 @@
 #endif /* MBEDTLS_DEPRECATED_REMOVED */
 #endif /* MBEDTLS_SSL_HW_RECORD_ACCEL */
 
+#if defined(MBEDTLS_SSL_DTLS_SRTP) && ( !defined(MBEDTLS_SSL_PROTO_DTLS) )
+#error "MBEDTLS_SSL_DTLS_SRTP defined, but not all prerequisites"
+#endif
+
 /*
  * Avoid warning from -pedantic. This is a convenient place for this
  * workaround since this is included by every single file before the