pppos-client: support for SIM7600 modem

Reuses most of the code from BG96. At this moment, this is not a
perfect support for other modules as the generic handlers for most
common modem functionality is not easily reusable. Refactoring to a
fully independent modem component will solve this.

Closes https://github.com/espressif/esp-idf/issues/5250

examples/protocols/pppos_client/components/modem/include/sim7600.h

@@ -0,0 +1,33 @@
+// Copyright 2015-2018 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.
+#pragma once
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include "esp_modem_dce_service.h"
+#include "esp_modem.h"
+
+/**
+ * @brief Create and initialize SIM7600 object
+ *
+ * @param dte Modem DTE object
+ * @return modem_dce_t* Modem DCE object
+ */
+modem_dce_t *sim7600_init(modem_dte_t *dte);
+
+#ifdef __cplusplus
+}
+#endif