As I understand the documentation for UNDEF in Makefile.vars (which makes not too much sense), the UNDEF variable should undefine object files to avoid their use in the final executable to avoid multiple definitions of functions.

In my case I want to override the gpio implementation of the native cpu with my own implementation in the board.

However, the UNDEF variable works the other way around. It is given as an extra parameter to the final gcc call in Makefile.include. This results in the explicit use of the undefined files by the linker!

I think UNDEF should be used somewhere around here. It should filter the objects.