net: event: Notify on interface being put down or up

It can be thus used to track the status of network interfaces in order
to act accordingly (no need to try sending things on network if the
interface is down for instance).

Change-Id: Ie160ef0dfdad6679d531d05a7abada96ad1de7db
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>