[dhcp] Add DHCP network device configurator

Provide an interface to DHCP via the generic network device
configurator mechanism.

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

src/net/udp/dhcp.c

 	ref_put ( &dhcp->refcnt );
 	return rc;
 }
+
+/** DHCP network device configurator */
+struct net_device_configurator dhcp_configurator __net_device_configurator = {
+	.name = "dhcp",
+	.start = start_dhcp,
+};