configs/releng/packages.x86_64: add squashfs-tools and udftools

• squashfs-tools can be used for system backup (with caveats though).
• By adding udftools the only yellow cell in https://wiki.archlinux.org/index.php/File_systems#Types_of_file_systems can be changed to green. Oh, and the tools can be used to format a UDF file system or something.