lwtunnel: add support for rpl segment routing

This patch adds support for rpl segment routing settings.
Example:

ip -n ns0 -6 route add 2001::3 encap rpl segs \
fe80::c8fe:beef:cafe:cafe,fe80::c8fe:beef:cafe:beef dev lowpan0

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: David Ahern <dsahern@gmail.com>