Use the IPv6 settings to construct the routing table, in a matter
analogous to the construction of the IPv4 routing table.

This allows for manual assignment of IPv6 addresses via e.g.

  set net0/ip6 2001:ba8:0:1d4::6950:5845
  set net0/len6 64
  set net0/gateway6 fe80::226:bff:fedd:d3c0

The prefix length ("len6") may be omitted, in which case a default
prefix length of 64 will be assumed.

Multiple IPv6 addresses may be assigned manually by implicitly
creating child settings blocks.  For example:

  set net0/ip6 2001:ba8:0:1d4::6950:5845
  set net0.ula/ip6 fda4:2496:e992::6950:5845

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