net: ipv6: add rpl sr tunnel

This patch adds functionality to configure routes for RPL source routing
functionality. There is no IPIP functionality yet implemented which can
be added later when the cases when to use IPv6 encapuslation comes more
clear.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>