mdev: use $DEVNAME in uevent sysfs file for device node name

516530c932bd17d87c9eb4347a490be051e495f4 uses $DEVNAME variable
for device node name. This is fine, but only works for hotplugging,
"mdev -s" will behave differently when DEVNAME and basename(path)
differ.

This patch extracts the DEVNAME from the uevent sysfs file in
make_device(), thus works for hot- and coldplugging; so using
the environment DEVNAME on hotplug events is no longer necessary.

Signed-off-by: Nikolaus Voss <n.voss@weinmann.de>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
Merge git://busybox.net/busybox