utils/generate-db-build.sh: initial support for split packages.

Symlinks are created in build tree for each split package name, pointing to the
package base directory.

This allows some sane handling of split packages. They aren't even rebuilt
multiple times for each pkgname, because building of the first pkgname actually
builds and installs all pkgnames (FIXME - a limitation), and other pkgnames are
then considered as uptodate.