ble/softdevice: add ble_nordic_softdevice feature

[haukepetersen]

Contribution description

This PR is a minor step on the path to make NimBLE the default solution for IP-over-BLE in RIOT, e.g. making it the default when building the gnrc_networking example.

But currently the softdevice is broken and there was some confusion about testing it also for the nrf52840, for which it was never verified to be working. This PR should clarify this situation once and for all :-)

Note about the approach: apparently the CPU_MODEL variable is not set when parsing the Makefile.features. At least I have not seen a more generic way in setting this feature explicitly for all nrf52832-based boards. Anything nicer is welcome :-)

Testing procedure

Build the gnrc networking example for the e.g. nrf52dk (-> should be all good), and for the e.g. nrf52840dk (-> should complain about the missing feature).

Issues/PRs references

partly related to #11559 and subsequently #11091

[aabadie]

At least I have not seen a more generic way in setting this feature explicitly for all nrf52832-based boards

This may also work: introduce a Makefile.nrf52832.features file in boards/common/nrf52 and include it in each nrf52832 boards features. That won't decrease the number of lines but provides a way to share nrf52832 specific features, so should a little more generic. I won't push toward this though and I think the current state of this PR is ok. Just needs testing.

[haukepetersen]

Yes, I thought about that option too, but found it a little overkill at first. But thinking about it now, its probably the cleanest solution after all -> will adapt.

[haukepetersen]

pushed the fix. Due to further complications through the nrf52xxxdk-common path I added the Makefile.nrf52832.features as additional include to the boards in question.

[aabadie]

Code changes are ok for me.
Also tested on nrf52dk and nrf52840dk and no problem found. The testing procedure described in this PR is not totally valid since softdevice is not pulled in for nrf52840 based board => no feature missing warning (these boards now use the nrf802154 driver directly).

Let's merge this.

ACK

[cladmi]

Note about the approach: apparently the CPU_MODEL variable is not set when parsing the Makefile.features. At least I have not seen a more generic way in setting this feature explicitly for all nrf52832-based boards. Anything nicer is welcome :-)

I found out about this PR while rebasing #11477. Makefile.features it is now the correct place to put CPU and CPU_MODEL as "Makefile.features: prerequisites for moving CPU/CPU_MODEL to boards/Makefile.features #11478" was merged (3 days ago)

The migration was not started due to being close to the release, and it will be announced on the mailing list too.

As it appears to be CPU_MODEL specific, when migrating this, I think I will after movingCPU and CPU_MODEL to board/.../Makefile.features, change to do a -include cpu/$(CPU)/Makefile.$(CPU_MODEL).features in the cpu/nrf52/Makefile.features.