cpu/nrf5x: decelare radio_nrfble feat for all nrfs

[haukepetersen]

Contribution description

The nrfble driver is working for all nrf-based CPUs (nrf51 and nrf52x), but the radio_nrfble feature is right now only and weirdly declared in boards/common/nrf52/Makefile.features. So the clean way would obviously be to declare that feature in cpu/nrf5x_common/Makefile.features, same as already done for radio_nrfmin.

Besides the structural cleanup, this change does furthermore enable skald to be build on nrf51-based platforms. This is intended and works as expected.

Testing procedure

make buildtest for examples/skald_x should build all boards that were build before, and also for all nrf51-based ones.

Further, at least one nrf51-based board should be flashed with a skald example and its behavior should should be verified with any BLE scanner (e.g. use your phone or examples/nimble_scanner on any nrf52-based board).

Issues/PRs references

none

[benpicco]

What about ble_nimble, is that specific to nrf52?

[haukepetersen]

Yes, at the moment it is, as there are some side effects to take into account when running nimble on the nrf51s. But I will look into this in a separate PR.

[haukepetersen]

The ble_nimble thins is also partly related to the cleanup in #11792

[benpicco]

Ok, then this is pretty straightforward.
Minor change, no regressions.

[haukepetersen]

thanks for the quick review!