sntp/lwip: Add some modes for time synchronization

Closes: IDF-236
Closes: https://github.com/espressif/esp-idf/pull/1668
Closes: https://github.com/espressif/esp-idf/pull/4103

components/lwip/include/apps/esp_sntp.h

@@ -0,0 +1,22 @@
+// Copyright 2015-2019 Espressif Systems (Shanghai) PTE LTD
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef __ESP_SNTP_H__
+#define __ESP_SNTP_H__
+
+#include "lwip/err.h"
+#include "lwip/apps/sntp.h"
+#include "sntp.h"
+
+#endif // __ESP_SNTP_H__