Revert "Remove unused TLS, NET, and X.509 files"

This reverts commit a4308b29a4.

doxygen/input/doc_tcpip.h

@@ -0,0 +1,46 @@
+/**
+ * \file doc_tcpip.h
+ *
+ * \brief TCP/IP communication module documentation file.
+ */
+/*
+ *
+ *  Copyright (C) 2006-2015, ARM Limited, All Rights Reserved
+ *  SPDX-License-Identifier: Apache-2.0
+ *
+ *  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.
+ *
+ *  This file is part of mbed TLS (https://tls.mbed.org)
+ */
+
+/**
+ * @addtogroup tcpip_communication_module TCP/IP communication module
+ *
+ * The TCP/IP communication module provides for a channel of
+ * communication for the \link ssltls_communication_module SSL/TLS communication
+ * module\endlink to use.
+ * In the TCP/IP-model it provides for communication up to the Transport
+ * (or Host-to-host) layer.
+ * SSL/TLS resides on top of that, in the Application layer, and makes use of
+ * its basic provisions:
+ * - listening on a port (see \c mbedtls_net_bind()).
+ * - accepting a connection (through \c mbedtls_net_accept()).
+ * - read/write (through \c mbedtls_net_recv()/\c mbedtls_net_send()).
+ * - close a connection (through \c mbedtls_net_close()).
+ *
+ * This way you have the means to, for example, implement and use an UDP or
+ * IPSec communication solution as a basis.
+ *
+ * This module can be used at server- and clientside to provide a basic
+ * means of communication over the internet.
+ */