busybox: enable CONFIG_FEATURE_MDEV_RENAME

Our default mdev.conf file in package/busybox has lines like:

  pcm.*           root:audio 660 =snd/
  event[0-9]+     root:root 640 =input/

that are used to create the device is the appropriate /dev
subdirectory. However, this feature requires
CONFIG_FEATURE_MDEV_RENAME to be enabled, otherwise mdev complains
with:

  mdev: bad line 25

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>