root@OpenWrt:~# cat mkbr.sh 
#!/bin/sh

ifconfig eth0 0.0.0.0
ifconfig ath1 0.0.0.0
brctl addbr br0
brctl addif br0 ath1
brctl addif br0 eth0
udhcpc -i br0 -f
ifconfig br0 > /tmp/ifconfig-br0