image.mk: move KDIR_TMP definition to a central location

Instead of each target defining it the same, move the KDIR_TMP
definition to include/image.mk. In addition Image/Build/SysupgradeNAND
already requires KDIR_TMP to be set, so it makes sense to have it
globally defined.

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46592 3c298f89-4303-0410-b956-a3cf2f4a3e73