It is not 100% clear if 4.12 is the last version the softdevice is able
to interact with, but newer kernel versions than 4.12 definitely
don't work (we tried 4.15 on Ubuntu 18.04 and 4.20 on a quite recent
Arch).
The reason I believe it's the 4.12 that last worked is that in the
documentation of the nRF5 SDK it is mentioned for something called
"legacy mode" [1]. However, we are using the nRF5 *IoT* SDK which
doesn't mention this legacy mode at all.
I don't know if we can update the `pkg` to the other SDK, so I just
updated the documentation accordingly for now.
[1]: https://www.nordicsemi.com/DocLib/Content/SDK_Doc/nRF5_SDK/v15-2-0/group__ble__6lowpan__config?9932#ga5285fd3eaf403092286f607cd66850f3
