mirrors: Add kernel.org sources mirror for downloads.yoctoproject.org

kernel.org now has a mirror of the downloads.yoctoproject.org sources
archive so include this in our mirrors list.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit f602b6c2)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>