As we are using kernel's DEVTMPFS, using udev-cache on top of it does
not add much advantage. Disabling it has the extra advantage of avoiding
writing the udev-cache tarball under '/etc' (which probably won't work
on a read-only system).
https://jira.digi.com/browse/DEL-3780
Signed-off-by: Javier Viguera <javier.viguera@digi.com>