busybox: provide /etc/mdev.conf if mdev is used

Without an mdev.conf file installed, mdev generates some /dev entries in
an incorrect or non-standard way. Some examples:
 1. /dev/null has permissions 660, but it should be accessible to normal
    users;
 2. alsa devices get created in /dev, not /dev/snd as is more common,
    and as the default value of BR2_PACKAGE_ALSA_LIB_DEVDIR suggests;
 3. event<N> files are created in /dev, not /dev/input.

This mdev.conf is a selection from the examples provided in the busybox
sources with minor tweaks.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/busybox/mdev.conf

@@ -0,0 +1,35 @@
+# null may already exist; therefore ownership has to be changed with command
+null		root:root 666 @chmod 666 $MDEV
+zero		root:root 666
+full		root:root 666
+random		root:root 444
+urandom		root:root 444
+hwrandom	root:root 444
+grsec		root:root 660
+
+kmem		root:root 640
+mem		root:root 640
+port		root:root 640
+# console may already exist; therefore ownership has to be changed with command
+console		root:tty 600 @chmod 600 $MDEV
+ptmx		root:tty 666
+pty.*		root:tty 660
+
+# Typical devices
+tty		root:tty 666
+tty[0-9]*	root:tty 660
+vcsa*[0-9]*	root:tty 660
+ttyS[0-9]*	root:root 660
+
+# alsa sound devices
+pcm.*		root:audio 660 =snd/
+control.*	root:audio 660 =snd/
+midi.*		root:audio 660 =snd/
+seq		root:audio 660 =snd/
+timer		root:audio 660 =snd/
+
+# input stuff
+event[0-9]+	root:root 640 =input/
+mice		root:root 640 =input/
+mouse[0-9]	root:root 640 =input/
+ts[0-9]		root:root 600 =input/