python-external-tarball: Modernise and rename

python-external-tarball: Modernise and rename

The external-python-tarball was intended to provide python for those users who
didn't have python 2.6. It has been unloved for a while so this refreshes
it a bit. Specifically:

* Include git and tar since these can have version issues on some systems
* Rename to buildtools-tarball to better reflect its purpose
* Remove the site-config file from the tarball, its pointless
* Prune down the environment file to just cover PATH and OECORE_NATIVE
  (which is needed to correctly install the tarball)
* Add missing grp module (used by do_package) from the unixadmin package

(From OE-Core rev: c8de009e96930709efe8f6aa4e65dd04a48081f2)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>