Unfortunately, it doesn't. My problems are:
a) In the case of my board, the hardware RNG is not part of the CPU but instead of the radio chip, which is accessed by the code in driver/at86rf2xx.
b) If I implement the hwrng interface in the at86rf2xx code, how does the samr21-xpro board learn of this? I couldn't find the code/configuration, where the at86rf2xx driver is attached to that board.
c) Can I simply put the periph_hwrng feature in the board's Makefile.features if I know that that interface is implemented in some of its parts?
Thanks a lot for your assistance, I just want to create a good patch for this, which can easily be integrated with the upstream code.