image: fix a typo in JFFS2OPTS to fix jffs2 rootfs generation

Due to a typo in the Makefile variable, mkfs.jffs2 is called
without the correct parameters.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt@37514 3c298f89-4303-0410-b956-a3cf2f4a3e73