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>