[ipv4] Support small subnets with no directed broadcast address

[ipv4] Support small subnets with no directed broadcast address

In a small subnet (with a /31 or /32 subnet mask), all addresses
within the subnet are valid host addresses: there is no separate
network address or directed broadcast address.

The logic used in iPXE to determine whether or not to use a link-layer
broadcast address will currently fail in these subnets.  In a /31
subnet, the higher of the two host addresses (i.e. the address with
all host bits set) will be treated as a broadcast address.  In a /32
subnet, the single valid host address will be treated as a broadcast
address.

Fix by adding the concept of a host mask, defined such that an address
in the local subnet with all of the mask bits set to zero represents
the network address, and an address in the local subnet with all of
the mask bits set to one represents the directed broadcast address.
For most subnets, this is simply the inverse of the subnet mask.  For
small subnets (/31 or /32) we can obtain the desired behaviour by
setting the host mask to all ones, so that only the local broadcast
address 255.255.255.255 will be treated as a broadcast address.

Originally-fixed-by: Lukas Stockner <lstockner@genesiscloud.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>