This simplifies building device / profile specific images, and allows
the build system to parallelize generating images
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43907 3c298f89-4303-0410-b956-a3cf2f4a3e73