add linux board project

board/Linux_Posix/mqtt_demo/readme.md

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+# How to run the demo in linux
+
+## step1
+make sure your develop environment.
++ `cmake` and version greater than 3.8.2
++ `gcc` `gdb` `make` is installed
+
+## step2
+configure your server and add a transmit rule
+
+reference [tencent cloud IoT guide?](../../../doc/8.TencentOS-tiny对接腾讯云IoTHub开发指南.md)
+
+additional thing, add a rule for send a message to the demo
+1. select filter topic is the ${demo device}/event and 
+SELECT * FROM ${demo device}/event
+2. select action type is Republish and topic is the ${demo device}/control
+
+the page like this
+_____________________________________________
+|规则引擎
+|--------------------------------------------
+|基本信息
+|规则名称  loop
+|规则描述  未填写
+|--------------------------------------------
+|筛选数据
+|字段  *
+|Topic  ${demo device}/event
+|条件
+|当前SQL语句是：
+|SELECT * ${demo device}/event
+|--------------------------------------------
+|行为操作
+|行为类型  数据转发到另一个Topic ( Republish )
+|Topic ${demo device}/control
+|  ...
+_____________________________________________
+
+
+## step3
+generate `mqtt_config.h` file to replace `./inc/mqtt_config.h`
+
+cd `tiny/tools/` directory, run python script
+```bash
+python3 mqtt_config_gen.py
+```
+
+then input your server configuration, generate `mqtt_config.h` file
+copy to `./inc/` replace old file
+
+## step4
+make `build` directory and compile in `build`
+
+```bash
+mkdir build && cd build
+cmake ..
+make
+```
+
+## step5
+run program !!
+
+```bash
+# in build directory
+./mqtt_demo
+```
+
+## other
+you can copy this demo to other path, but if you want do it,
+you need modify `CMakeLists.txt`. find line
+
+```cmake
+set(TINY_ROOT ../../../)
+```
+
+and modify `path-to-tinyos`
+```cmake
+set(TINY_ROOT path-to-tinyos)
+```