image.mk: replace all Build/netgear-chk with a generic implementation

Instead of letting each target define it themselves, create on in
include/image.mk and let the targets use it.

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46596 3c298f89-4303-0410-b956-a3cf2f4a3e73