Okay
in this thread:
https://community.onion.io/topic/1134/guide-to-cross-compilation-for-omega2/7
I read that it is the LEDE Toolchain with musl that I have to use to cross-compile sources
I also found this link with a tutorial on how to cross-compile custom images:
https://wiki.onion.io/Tutorials/Cross-Compile
With the new Toolchain I have managed to cross compile the OpenCV sources for the Omega2
But I still have no idea how to cross compile the kernel modules 
or even where I can find the kernel sources
