[ipv6] Add IPv6 network device configurator

Include IPv6 within the generic network device configurator
mechanism.  The IPv6 configurator will send a router solicitation and
wait for a router advertisement to be received.  (As per RFC4861
section 6.3.7, we do this even if advertisements have been received
prior to sending the router solicitation.)

Signed-off-by: Michael Brown <mcb30@ipxe.org>