Improve cpe examples

[wt]

Summary

• Make the cpe usb_serial example use rtic. This allows the removal of the static muts, which are not legal in edition 2024.
• Add an blinky example that uses rtic.

Checklist

• [X] All new or modified code is well documented, especially public items
• [X] No new warnings or clippy suggestions have been introduced - CI will deny clippy warnings by default! You may #[allow] certain lints where reasonable, but ideally justify those with a short comment.

[wt]

I isolated the feature change that adds the rtic support for this board, so this should only land after #966.