openwrt: don't build ALL packages

While the core images etc are not reproducible, there is little point to
excessively build packages. It takes a long time and currently even
crashes the build entirely.

Disable it for now and add a second role later which actually rebuilds
packages from community feeds.

Signed-off-by: Paul Spooren <mail@aparcar.org>
Signed-off-by: Holger Levsen <holger@layer-acht.org>