rfkill: move wait_for_initialized() to shared/

rfkill: move wait_for_initialized() to shared/

The function interface is the same, except that the output pointer may be NULL.

The implementation is slightly simplified by taking advantage of changes in
ancestor commit 'sd-device: attempt to read db again if it wasn't found', by
not creating a new sd_device object before re-checking the is_initialized
status.

v2:
- In v1, the old object was always used and the device received back from the
  sd_device_monitor_start callback was ignored. I *think* the result will be
  equivalent in both cases, because by the time we the callback gets called,
  the db entry in the filesystem will also exist, and any subsequent access to
  properties of the object would trigger a read of the database from disk. But
  I'm not certain, and anyway, using the device object received in the callback
  seems cleaner.