scripts/install-buildtools: bump to 3.1 release by default

By default, use the extended buildtools installer from the
Yocto Project 3.1 "dunfell" release.

Signed-off-by: Tim Orling <timothy.t.orling@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Steve Sakoman <steve@sakoman.com>