kernel: enable SRv6 support by enabling lwtunnel

kernel: enable SRv6 support by enabling lwtunnel

Enable the ability to use segment routing based on IPv6. It allows the
packet to specify a path that the packet should take through the
network.

Lwtunnel allow an easy encapsulation of a package. You can just install
ip-full package and use it:

  ip -6 route add  2003::/64 dev eth0 encap seg6 mode encap \
    segs 2001::1,2002::2

An IPv6 package looks like this:
  [IPv6 HDR][IPv6 RH][IPv6 HDR][Data...]

Netifd support:
  https://git.openwrt.org/?p=project/netifd.git;
     a=commit;h=458b1a7e9473c150a40cae5d8be174f4bb03bd39

Increases imagesize by 24.125 KiB. Therefore, only enable for devices
with enough flash.

Signed-off-by: Nick Hainke <vincent@systemli.org>