Disable parallel building because it leads to random compile errors. Details: OpenWrt uses Buildroot and the buildroot documentation notes: "You should never use make -jN with Buildroot: top-level parallel make is currently not supported. Instead, use the BR2_JLEVEL option to tell Buildroot to run the compilation of each individual package with make -jN." This means parallel building of the OpenWrt tools and toolchain is not supported. But parallel building of the OpenWrt packages is and can be enabled using the BR2_JLEVEL option. I tried this but sadly without luck. Seems like packages do not get build in parallel when using BR2_JLEVEL. Maybe because the OpenWrt buildroot is wrapped into some Gluon stuff...

Signed-off-by: Clemens John <clemens.john@floh1111.de>