`UsbSerialJtag`s `core::fmt::Write` impl is blocking

[dimpolo]

This means that writeln!(&mut serial, "Hello") will block if USB is not connected.

Should we change this? Should we copy esp-printlns implementation? https://github.com/esp-rs/esp-println/blob/ea55c308b56a737d1ce20e7acca49c6acc45b305/src/lib.rs#L99-L195