rpb-inc: avoid to duplicate the rootfs tarball by generating tar.{gz,xz}
rpb-inc: avoid to duplicate the rootfs tarball by generating tar.{gz,xz}

Keep tar.xz since it's much smaller.

Signed-off-by: Fathi Boudra <7cd6e55cc07a66bc71d7901a4cddfe6c22aab659@linaro.org>

96boards/meta-rpb,fboudra/meta-rpb,fboudra/meta-rpb,96boards/meta-rpb,fboudra/meta-rpb