udev-extraconf: not automount network block devices

udev-extraconf: not automount network block devices

The nbd devices are set up loop devices, we first have dead device
nodes, then later we attach stuff behind them. At the time of the udev
creation event, nothing works at nbd devices.

This avoids unnecessary boot error msg:
| nbd0: Attempted send on closed socket
| end_request: I/O error, dev nbd0, sector 0

Signed-off-by: Ting Liu <ting.liu@freescale.com>