ar71xx: Generate sysupgrade images for OpenMesh devices

ar71xx: Generate sysupgrade images for OpenMesh devices

Some OpenWrt based firmwares like Gluon expect that a sysupgrade image
exists when a device firmware can be updated via sysupgrade. This image
wasn't created until now because OpenMesh devices use the same image for
factory and sysupgrade flash. Copying the image from *factory.bin to
*sysupgrade.bin is therefore enough to make the sysupgrade functionality
visible.

Reported-by: Matthias Schiffer <mschiffer@universe-factory.net>
Signed-off-by: Sven Eckelmann <sven.eckelmann@open-mesh.com>