[sample] add TCP echo server and client (openthread#17)

README.md

@@ -112,3 +112,10 @@ A sample config is given below:
 `CLOUDIOT_CERT` is the first certificate from [https://pki.google.com/roots.pem](https://pki.google.com/roots.pem)
 
 You can use command `test mqtt` to test connect and publish a message to Google Cloud IoT core.
+
+### TCP echo server and client
+Commands:
+- `tcp_echo_server port` starts TCP echo server on given `port`.
+- `tcp_connect ipaddr port` connects to given TCP server.
+- `tcp_send size count` sends `count` packets with given `size` to connected TCP server. At the end it prints statistics.
+- `tcp_disconnect` disconnects from TCP server.

src/apps/test/CMakeLists.txt

@@ -3,6 +3,7 @@ cmake_minimum_required (VERSION 3.7)
 add_library(test_app
     ${CMAKE_CURRENT_SOURCE_DIR}/http.c
     ${CMAKE_CURRENT_SOURCE_DIR}/mqtt.cpp
+    ${CMAKE_CURRENT_SOURCE_DIR}/tcp_utils.c
     ${CMAKE_CURRENT_SOURCE_DIR}/user.c)
 
 target_link_libraries(test_app