generators: Skip potentially problematic root-level entries

Since we replicate the creation of some symlinks in the rootfs-builder-debos repo, which is run before system-image-server gets to touch the tarball, we need to make sure duplicate Android compatibility symlinks and their target directory (/android) are skipped when system-image-server creates the target tarball for consumtion by our non-default tar setups in the recovery (busybox tar).

Track what we add at the top level and make sure to not create yet another variant.