When a network device is specified to tcpip_tx() or it's children, treat

it as a fallback network device rather than an explicitly forced network
device.

Clear routing table entries before re-attempting DHCP.

src/usr/dhcpmgmt.c

@@ -58,6 +58,9 @@ int dhcp ( struct net_device *netdev ) {
 		dhcp_options = NULL;
 	}
 
+	/* Clear any existing routing table entry */
+	del_ipv4_address ( netdev );
+
 	/* Issue DHCP request */
 	printf ( "DHCP (%s %s)...", netdev->name, netdev_hwaddr ( netdev ) );
 	memset ( &dhcp, 0, sizeof ( dhcp ) );