net: rpl: Do not do neighbor discovery for RPL network

Doing neighbor discovery in RPL network is not necessary and
that can be disabled in RPL nodes. Unfortunately the border
router needs to have ND enabled as it has also non-RPL network
interfaces in use. So in this case, mark RPL node as always
reachable.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>