octeon: fix image build

The generic sysupgrade image attempted to use the wrong filesystem type due
to premature variable interpolation leading to the following error on the
buildbot system:

    cp: cannot stat `.../root.squashfs': No such file or directory
    make[4]: *** [.../tmp/lede-octeon-generic-ext4-sysupgrade.tar] Error 1

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

target/linux/octeon/image/Makefile

@@ -16,7 +16,7 @@ define Device/Default
   KERNEL := kernel-bin | strip-kernel | patch-cmdline
   IMAGES := sysupgrade.tar
   FILESYSTEMS := squashfs
-  IMAGE/sysupgrade.tar := tar-file $$(FILESYSTEMS)
+  IMAGE/sysupgrade.tar := tar-file $$$$(FILESYSTEMS)
 endef
 
 define Build/tar-file