Add network scripting folders to fs/skeleton

Those folders are currently created using makedevs according to the
specifications in device_table.txt. However, as makedevs is no longer
executed when dynamic device creation methods are selected (devtmpfs,
udev, mdev), those folders must be created differently. We choose to
put them directly into the default filesystem skeleton.

Signed-off-by: Yegor Yefremov <yegor_sub1@visionsystems.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>