opendingux: Disable usb0 and wlan0 when shutting down

The "ifdown -a" command will shut down all the network interfaces that
are marked as "auto" within /etc/network/interfaces. However, neither
usb0 nor wlan0 were marked as auto, so they were never properly
disabled.
opendingux: Wait for hwclock to complete before exiting ntpd script

While it was working fine when the interface was disabled at runtime,
hwclock couldn't save the clock properly on shutdown.