image: fix jffs2(_nand) image generation

Variables dependend on JFFS2_BLOCKSIZE and NANDBLOCK_SIZE are used
for template generation, so need to be present before inclusion of
image.mk in target image Makefiles.
So move all declarations to before any includes.

Fixes: r42878 ("image.mk: clean up and parallelize mkfs calls")
Signed-off-by: Jonas Gorski <jogo@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46564 3c298f89-4303-0410-b956-a3cf2f4a3e73