Purging unlisted packages

[Andreas Heinlein]

Hello,

I have a question regarding "take-over" of clients not initially
installed with FAI. Is it possible to purge any packages *not* listed in
package_config during softupdate? I have read that one should not just
remove a package from the list that was previously installed, but
instead prepend it with a '-' sign. But what if you do not know exactly
which packages were previously installed and want to just remove any
installed but unlisted package?

Thank you,
Andreas