make: Allow for true pseudo-submodules
make: Allow for true pseudo-submodules

When providing a pseudo-submodule for a module already using the
`SUBMODULES` mechanism to provide submodules, it is not possible to
create a true pseudo-module as submodule (i.e. one without any code on
its own), since the build system currently always expects there to be a
C file `module_submodule.c`.

This removes this requirement.

kaspar030/RIOT,jasonatran/RIOT,jasonatran/RIOT,kYc0o/RIOT,basilfx/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,kYc0o/RIOT,miri64/RIOT,kYc0o/RIOT,ant9000/RIOT,kaspar030/RIOT,basilfx/RIOT,RIOT-OS/RIOT,miri64/RIOT,jasonatran/RIOT,ant9000/RIOT,kYc0o/RIOT,authmillenon/RIOT,basilfx/RIOT,authmillenon/RIOT,kaspar030/RIOT,kaspar030/RIOT,kYc0o/RIOT,OlegHahm/RIOT,basilfx/RIOT,ant9000/RIOT,authmillenon/RIOT,ant9000/RIOT,jasonatran/RIOT,miri64/RIOT,miri64/RIOT,OlegHahm/RIOT,basilfx/RIOT,OlegHahm/RIOT,authmillenon/RIOT,miri64/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,ant9000/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,jasonatran/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT