Print USB timestamps nicely

[jonathanpallant]

Instead of printing a raw Duration, and until knurling-rs/defmt#911 is resolved, we could print "{=u64:tus}", duration.as_micros() as u64. It would look nicer in the logs.

[jonathanpallant]

Or we could rely on the timestamp support built into defmt, and show people how to pass a log-format that prints timestamps.

That involves looking at defmt-decoder to see what log formats it has built in (and at probe-rs to see what log formats it supports by default), and then putting a better default in our .cargo/config.toml file's runner line. And of course implementing defmt::timestamp using dk::uptime(), or some new function dk::uptime_as_microseconds() which might help avoid some unnecessary multiplication and division.

[jonathanpallant]

Fixed in #152