overlayfs: all overlays unit

Application can depend on several overlayfs mount points. Provide a
systemd unit application can depend on to make sure all overlays are
mounted before it is started to avoid any race conditions

Signed-off-by: Bruno Knittel <Bruno.Knittel@bruker.com>
Signed-off-by: Vyacheslav Yurkov <uvv.mail@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>