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>

package/busybox/S10mdev

@@ -6,6 +6,7 @@
 case "$1" in
   start)
 	echo "Starting mdev..."
+	echo /sbin/mdev >/proc/sys/kernel/hotplug
 	/sbin/mdev -s
 	;;
   stop)