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>