* Add diagnostics module in OpenThread
- provide the same diagnostics interface for both CLI and NCP usage
- implement common diagnostics features based on existing platform interface defined in 'include/platform/'
- other more platform specific diagnostics features will be processed under platform layer
- update CLI interface to support diagnostics feature
- update both Posix and CC2538 platform to support diagnostics feature
* Add diagnostics module unit test
- move platform.h from "examples/platform" to "include/platform"
- add test_diag.cpp to test diagnostics module
* Add a configuration option that would enable/disable diagnostics module
Add --enable-diag configuration option to enable/disable diagnostics module when building OpenThread.
* Redefine otEnable/otDisable to just initialize/uninitialize the OpenThread stack.
* Remove otInit API since it is replaced by otEnable.
* Add otInterfaceUp/otInterfaceDown APIs to bring up/down IPv6 interface.
* Add otThreadStart/otThreadStop APIs to start/stop Thread protocol operation.
* Updated NCP implementation to utilize new APIs.
Jonathan Hui