samples: net: rpl: Enhance RPL border router application

Add CoAP client to RPL BR application. Which can be used to
control leds and get rpl related information from leaf nodes
though CoAP messages. Build toplogy matrics and send it to
Web UI.

Enhanced Web UI components (Interfaces, RPL, Neighbors, Routes
and Topology tabs). Added buttons to control leds on lead nodes.

Added JSON support to handle led on/off requests from UI.

Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>