ar71xx: Allow OpenMesh CE images with more than 3 files

The CE image format used by OpenMesh can contain extra blocks which
are not used for flashing. Only the first three embedded images
(fwupgrade.cfg, kernel, rootfs) are required in this order to successfully
flash an image via sysupgrade. All extra embedded images should be ignored
for the available devices.

Signed-off-by: Sven Eckelmann <sven.eckelmann@open-mesh.com>