two new tools:
create_libretro_tarballs.sh: creates a tarball of already existing
folder with cloned repository for future re-use
get_libretro_tarball.sh: retrieves a tarball from a mirror in case a
specific version cannot be checked-out by the buildsystem
create_libretro_tarballs.sh should be run by the mainteners on regular
basis, so there are backups of most versions of used libretro
packages/cores
get_libretro_tarball.sh should be run by a builder in case the build
errors due to non-existent hash in cloned repository
