Simplify udev startup script

The S10udev startup script did useless things such as mounting sysfs,
mounting a tmpfs filesystem in /dev/, etc. Those things are already
done during in the default inittab.

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