customize: firstboot: make sure to run Linux scripts only once

If a script does not finish, hangs, etc, it would be executed again at
the next boot, since the injected firstboot.sh helper removes it only
after it finished.

Before executing a script, move it to another internal subdirectory
(scripts-done) and execute it from there, so it is not run again by
firstboot.sh.  The downside is that now scripts are executed only once
at all, so if a script fails it will not be attempted at the next boot.

Also, remove all the files found in scripts-done, as they have been run
(or at least attempted) in a previous boot.

This fixes RHBZ#1159651.