I think ${JFFS2_ENDIANNESS} has no effect (jffs2 will always be generated as big endian, the default of mkfs.jffs2).

meta/classes/image_types.bbclass defines default arguments for mkfs.jffs2:

EXTRA_IMAGECMD_jffs2 ?= "--pad ${JFFS2_ENDIANNESS} --eraseblock=${JFFS2_ERASEBLOCK} --no-cleanmarkers"

Those are inserted into 
IMAGE_CMD_jffs2 = "mkfs.jffs2 --root=${IMAGE_ROOTFS} --faketime --output=${IMGDEPLOYDIR}/${IMAGE_NAME}${IMAGE_NAME_SUFFIX}.jffs2 ${EXTRA_IMAGECMD}" 

According to "man mkfs.jffs2":

-p, --pad[=SIZE]
 Pad output to SIZE bytes with 0xFF.  If SIZE is not specified, the output is padded to the end of the final erase block.

pad expects a size argument.

mkfs.jffs2 now silently accepts -b or -l and converts it to 0. Hence the argument is already consumed and will not have the intended effect.

Workaround: Overwrite EXTRA_IMAGECMD_jffs2 in image-recipe