mvebu: add ClearFog .tar.gz bundle

The previous image building code rework removed the rootfs.tar.gz with embedded
kernel and dtb build artifact which is required to build suitable SD images.

Reintroduce a .tar.gz artifact locally which embeds kernel and dtb, similar to
how the old code handled it.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>