Basic C interface to the Rubble capsule

[daboross]

This adds an interface to the rubble capsule added in https://github.com/tock/tock/pull/2036.

The rubble capsule currently exposes very little functionality, and this interface is thus equally minimalistic. I've based it, and the test app, on a reduced version of the existing ble_advertising interface and test app.

[phil-levis]

Sorry, catching up with this now.

Why do we want a Rubble BLE interface and a BLE interface? I.e., why doesn't Rubble just implement the API in ble.h? Is this so we can have two implementations in parallel?

[alistair23]

I would say the main benefit of having two interfaces is that we can then support both implementations in parallel.

I think we should work on removing the current BLE implementation and replace it with Rubble, but that won't happen straight away even if at all.

[ppannuto]

I think @phil-levis's question is in the same vein as the discussion currently going on at https://github.com/tock/tock/pull/2233. We've historically held the libtock-c merges until the kernel merges are ready/done -- but I think this is fine once the other half is ready.

[ppannuto]

Rubble project EOL'd mid-2022 :(.