Thanks, now I have two different opinions on this issue 
As I mentioned the build continues if the .mk-file of Superuser is renamed. Renaming or deleting the mk-file of the FP-build-tree will lead to early errors on includes. However I would like to be sure that .mk-files of any names are integrated properly. Or even better, to learn about how to work around such things. I mean this kind of problem should be common. Is there no possibility to “tell” the system-.mk-file to call the Superuser-.mk-file aswell?