opendingux: Change metric of interface usb0

When both usb0 and wlan0 are present on the system, it has been
reported that both having the same metric creates a huge packet
drop on the wireless interface.
opendingux: busybox config: Enable 'rfkill'
opendingux: Modify wlan scripts to use rfkill
opendingux: Modify 'ap' script to use rfkill