Hello Mathias,
The prebuilt images are older than two years. My advice would be to build coreboot from source, from either master or 4.8.1 (4.8 is broken). To use the fn/ctrl swap, enabling "Use CMOS for configuration values" in Kconfig is needed.
Note that using nvramtool as Evgeny stated should be possible with the old image though.
Regards,
Angel Pons