packet lib: icmpv6: add nd_router messages

add ICMPv6 sub encoder/decoder class for Router Solicitation and
Router Advertisement messages.

add ICMPv6 sub encoder/decoder class for Neighbor discovery
Prefix Information Option.

Signed-off-by: Ygor Amaral <yabls@cin.ufpe.br>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>