Add RPL border router functionality to Zephyr

[zephyrbot]

Reported by Andrei Laperie:

In order to route packets to/from 15.4 mesh using another networking interface (Ethernet for example), border routing functionality needs to be added to Zephyr.
The sample hardware configuration would be FRDM K64-F board with NXP-MCR20A adapter and Ethernet connected

(Imported from Jira ZEP-2391)

[rveerama1]

samples/net/rpl_border_router #5035 is the example for RPL Border Router functionality.