Quoting Dr. Nikolaus Klepp (dr.klepp@???):
> Hi !
> 
> Am Montag, 26. Juni 2017 schrieb karl@???:
> > From https://busybox.net/downloads/BusyBox.html
> > mdev
> >     mdev [-s]
> >             -s      Scan /sys and populate /dev during system boot
> >     It can be run by kernel as a hotplug helper. To activate it:
> >     echo /bin/mdev >/proc/sys/kernel/hotplug It uses /etc/mdev.conf
> >     with lines [-]DEVNAME UID:GID PERM [>|=PATH] [@|$|*PROG]
> > 
> > What mdev probably is missing is libudev which some programs whishes to 
> > use.
> 
> /proc/sys/kernel/hotplug is missing in debian & devuan kernel since
> some years now. 
Ummm....
/proc/sys/kernel/hotplug appears to be a /proc key you _create_.
As to whether hotplug support is present in your kernel, what does 
'zless /proc/config.gz | grep CONFIG_HOTPLUG=' say?
https://stackoverflow.com/questions/2111278/correct-linux-kernel-configuration-for-hotplug
https://serverfault.com/questions/303085/what-is-mdev-and-how-to-dev-nodes-get-created-in-linux