Add diagnostics module in OpenThread (#343)

* 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.

tests/unit/test_ncp_buffer.cpp

@@ -40,6 +40,10 @@ extern"C" void otSignalTaskletPending(void)
 {
 }
 
+extern "C" void otPlatDiagAlarmFired()
+{
+}
+
 enum
 {
     kTestBufferSize = 101,         // Size of backed buffer for NcpFrameBuffer.