Previously udev_wait_event also allowed to look for different state
changes ("add", "remove", "change"). Now that udev_wait_event is now for
adding devices, create a new helper function for change events only.

The musca actions will need to look for "add" and "change" events.

Change-Id: I60bfc90c4c7688972717291af67083037ecf8da5
Signed-off-by: Dean Birch <dean.birch@arm.com>