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@36860 3c298f89-4303-0410-b956-a3cf2f4a3e73
kirkwood: convert to use the new jffs2_nand

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

git-svn-id: svn://svn.openwrt.org/openwrt@36861 3c298f89-4303-0410-b956-a3cf2f4a3e73
omap24xx: convert to use the new jffs2_nand

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

git-svn-id: svn://svn.openwrt.org/openwrt@36862 3c298f89-4303-0410-b956-a3cf2f4a3e73
target: don't build jffs2 nor images by default

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

git-svn-id: svn://svn.openwrt.org/openwrt@36863 3c298f89-4303-0410-b956-a3cf2f4a3e73