set PACKAGE_FILES environment variable for postbuild commands

[jaxvanyang]

I'm trying to use repo-add to update my repo db after build, so I want to know file names of the new built package files. Writing a script to find them is a little diffcult, but lilac has a simple API to do it! So I made this change, PACKAGE_FILES are space-seperated file names and their parent paths are stripped because the final file path may be destdir or staging dir.

I think this is a good adition to post scripts, for example, I can now use a simple script below to add new packages:

for i in $PACKAGE_FILES; do
  repo-add /path/to/repo.db "$destdir/$i"
done

[lilydjwg]

Thanks!