package/busybox: support automatic module loading with mdev

package/busybox: support automatic module loading with mdev

A recently discussed on the mailing list:
http://lists.busybox.net/pipermail/buildroot/2016-February/154189.html

Our mdev configuration currently doesn't handle module loading. Fix that by:

- Telling mdev to run modprobe on hotplug events providing MODALIAS

- Adjust the init script to handle coldplug modalias events (E.G. modules
  for which the devices were already present before mdev was added as the
  hotplug handler). mdev -s should arguable handle this, but it doesn't.

Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>