@dc42 Are you sure it built all of them? For me it builds the active one (which I didn't even notice) plus the one that's linked.
For instance, I went through the CoreNG, RRFLibraries and FreeRTOS projects and made SAM3X* the active config but linked all the SAM4E* configs. When I build RepRapFirmware, it builds the SAM3X* configs plus the SAM4E* configs but that's it. Since those libs are tiny, there's no real downside to building the extra SAM3X* configs and RRF links to the correct SAM4E* versions. Well, "correct" for me since I build Duet2_RTOS.