target: split jffs2 NAND out of jffs2

Make jffs2_nand a separate option so this can be triggered without forcing
jffs2 images for nor targets.

Adds a new NAND_BLOCKSIZE variable that allows setting the generated layout
in <page_size>:<block_size> pairs.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@36860 3c298f89-4303-0410-b956-a3cf2f4a3e73