Previouly, the udev-cache has no real effect even if it's installed
into the system. The key problem here is that at first boot, the
/etc/dev.tar is not present, thus resulting /dev/shm/udev.cache not
created on first boot even if udev-cache is enabled.
This patch fixes this problem. The /dev/shm/udev.cache will be created
if necessary, that is, on first boot or when some part of the system is
changed. In the latter case, the udev cache may not be valid.
