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>

target/linux/ar71xx/image/legacy.mk

@@ -919,6 +919,9 @@ define Image/Build/OpenMesh
 		"$(BUILD_DIR)/fwupgrade.cfg-$(4)" "fwupgrade.cfg" \
 		"$(KDIR_TMP)/vmlinux-$(2).uImage" "kernel" \
 		"$(KDIR)/root.$(1)" "rootfs"
+	if [ -e "$(call factoryname,$(1),$(2))" ]; then \
+		cp "$(call factoryname,$(1),$(2))" "$(call sysupname,$(1),$(2))"; \
+	fi
 endef