[ipv6] Separate the concepts of prefix and address creation

Allow for IPv6 routing table entries to be created for an on-link
prefix where a local address has not yet been assigned to the network
device.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[dhcpv6] Allow stateful DHCPv6 to apply obtained IPv6 addresses

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