[dhcp] Request broadcast responses when we already have an IPv4 address

[dhcp] Request broadcast responses when we already have an IPv4 address

FCoE requires the use of multiple local unicast link-layer addresses.
To avoid the complexity of managing multiple addresses, iPXE operates
in promiscuous mode.  As a consequence, any unicast packets with
non-matching IPv4 addresses are rejected at the IPv4 layer (rather
than at the link layer).

This can cause problems when issuing a second DHCP request: if the
address chosen by the DHCP server does not match the existing address,
then the DHCP response will itself be rejected.

Fix by requesting a broadcast response from the DHCP server if the
network interface already has any IPv4 addresses.

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