Add license headers, update namespaces

esp_modem/src/esp_modem_dte.cpp

@@ -1,7 +1,25 @@
+// Copyright 2021 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.
+
 #include "cxx_include/esp_modem_dte.hpp"
 #include <string.h>
 #include "esp_log.h"
 
+using namespace esp_modem;
+
+const int DTE_BUFFER_SIZE = 1024;
+
 DTE::DTE(std::unique_ptr<Terminal> terminal):
         buffer_size(DTE_BUFFER_SIZE), consumed(0),
         buffer(std::make_unique<uint8_t[]>(buffer_size)),