If someone is interested, I've setup a web page which describes how to
build a DCF77 (radio) HID USB RTC using an Atmel ATmega32U4 (e.g. an
Arduino Leonardo, Teensy or another clone), which uses the
rtc-hid-sensor-time driver to function as a stratum 0 time source:
http://ahsoftware.de/dcf77-hid-usb-rtc/
Regards,
Alexander