buildroot

busybox: register mdev as hotplug helper when selected

It's also done in the kernel configuration, however users may be using
some other pre-built kernel and miss functionality like firmware
loading.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>