Update loopback fw

[robamu]

Based on #226 .

Like expected, not exactly trivial, but definitely a good learning opportunity. Let's see if async can actually simplify the complexity here.

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[robamu]

Waiting for #226, but some notes:

• Introduce shared BSP
• Implementation did not become less complex, but some of the low-level details are hidden now
• USB ACM is currently a separate task. For the dongle, this is more or less pointless at it will always used as a USB device. But it's a a good thing when running on the DK: The USB ACM is now optional. However, a pipe between the USB ACM and radio task now is now required to send data from the radio task via ACM.
• I tried to make sleeping work when there is nothing to do using https://rtic.rs/2/book/en/by-example/app_idle.html. However, something is not working and the loopback does not work. Did not do any in-depth investigation into that yet.
• Simplified running on the dk board a bit