[ipv6] Handle IPv6 option length correctly

The IPv6 option length field represents the length of the option data
field, not the overall length of the option.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[ipv6] Extract link layer addresses from router advertisements

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[ipv6] Support stateless address autoconfiguration (SLAAC)

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