convert-overrides.py: show processed file and version of this script

* on bigger layer this script takes couple minutes without showing any progress
* add a version to reference it in the conversion commit (to easily figure out
  if it's worth re-converting the layer).

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>