* 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.
* Enhance ping command
* add Doxygen for timestamp in EchoReplyHandler
* fix function params name style
* use echo request containing timestamp instead of maintaining internal states
* add mac layer counter
* add more counters and Doxygen for MAC counters
* have a seperate counter for acked data transmission and refine the code based on the comments
* refine counters for acked and unacked transmissions
* fix check test error
* refine code style and use VerifyOrExit to exit
* move all events tracker to tx/rx done task, and add more counters
* fix other error counter
* refine counters structure to be clearer
* add Doxygen for the added thread errors
* fix conflicts
* Add an LQI field to the radio packet structure.
* Add an LQI field to the ThreadMessageInfo.
* Add an LQI field to the scan result.
* Print LQI value in CLI scan result.