net: ipv6: Incorporate IPv6 support into u-boot net subsystem

Add net_ip6_handler (an IPv6 packet handler) into net_loop. Add
neighbor discovery mechanism into network init process. That is the
main step to run IPv6 in u-boot. Now u-boot is capable to use NDP and
handle IPv6 packets.

Signed-off-by: Viacheslav Mitrofanov <v.v.mitrofanov@yadro.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>