grml-live's buildd part already supports the IGNORE class to list packages that 
should NOT be installed even though they are list in other classes.

But: depending on the amount of packages to be installed this might go wrong 
due to the way FAI install packages in bundles of $MAXPACKAGES.

I'd like to have a *reliable* way to blacklist packages which are known to be 
broken (e.g. in unstable, as with slapd, ccrypt,... nowadays) and should be 
ignored until they are known to be working again.