net/mgmt: Add a function to wait on a event synchronously

Instead of creating a handler and a related callback structure on an
event_mask: net_mgmt_event_wait() can be used to wait synchronously on
such event_mask. The core mgmt part will seamlessly reuse the struct
net_mgmt_event_callback so the whole internal notification mechanism is
using the same code.

Change-Id: I426d782c770e75e5222aa3c5b703172b1f1f2e5e
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>