[ipv4] Extend routing mechanism to handle non-default routes

[ipv4] Extend routing mechanism to handle non-default routes

Extend the definition of an IPv4 routing table entry to allow for the
expression of non-default gateways for specified off-link subnets, and
of on-link secondary subnets (where we can send directly to the
destination address even though our source address is not within the
subnet).

This more precise definition also allows us to correctly handle
routing in the (uncommon for iPXE) case when multiple network
interfaces are open concurrently and more than one interface has a
default gateway.

The common case of a single IPv4 address/netmask and a default gateway
now results in two routing table entries.  To retain backwards
compatibility with existing documentation (and to avoid on-screen
clutter), the "route" command prints default gateways on the same line
as the locally assigned address.  There is therefore no change in
output from the "route" command unless explicit additional (off-link
or on-link) routes are present.

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