overlays: Add sdtweak features for network booting

It has been observed that a Pi with no SD card will poll the interface
continuously, using up to 10% CPU. Add some new parameters to the
sdtweak overlay to control this behaviour:

poll_once   Only look for a card once, at boot time. If none is found
            then the interface is effectively disabled.

enable      Set to "off" or "no" to completely disable the interface.

See: https://github.com/raspberrypi/linux/issues/2567



Signed-off-by: Phil Elwell <phil@raspberrypi.org>