Remove obsolete mkdir creation in autoyast

The dir is already created in the task.
Restrict the exported Autoyast directory to the user

As we discussed we don't want the exported directory to be
readable for all.
Provide note about Autoyast export permissions

Also add information to the readme how to change the permission.
Merge pull request #444 from SUSE/make_autoyast_export_private

Make autoyast export private