Example for stm32wb controller

[lulf]

• Create bt-hci implementation in https://github.com/embassy-rs/embassy/tree/main/embassy-stm32-wpan
• Add example
• Update README.md

[chrysn]

Thanks for the pointers, and sorry for the noise – no clue why my search for STM32 didn't find that.

trouble requires the bt-hci crate interface to be implemented, so I think the task here is to implement the bt-hci crate in https://github.com/embassy-rs/embassy/tree/main/embassy-stm32-wpan (which already implements stm32wb-hci)

There's one mini item to do here once the right traits are implemented: list it as supported backends. (Rust's "just provide some impl of the trait" scheme is great, but it results in terrible discoverability of what might work unless the READMEs point it out)