[ipv6] Reduce time spent waiting for router discovery

[ipv6] Reduce time spent waiting for router discovery

Now that IPv6 is enabled by default for UEFI builds, it is important
that iPXE does not delay unnecessarily in the (still relatively
common) case of a network that lacks IPv6 routers.

Apply the timeout values used for neighbour discovery to the router
discovery process.

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