rootfs-overlay: use a make foreach instead of a shell for

Makes the code uniform with the post-build and post-image implementation
(which is slightly simpler and, presumably, more efficient).

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>