rtt: fix hang when aux serial not read

[koendv]

Detailed description

This PR silently drops rtt output if the aux serial is unresponsive for 10ms.
Fixes #1488 RTT output hangs the BMP when not read by host.
Tested on blackpill-f411.

A more general solution would be to append the output of usb serial, traceswo decode, rtt, and redirect_stdout to a single circular buffer, and use the usb tx interrupt to trigger usb writes.

Your checklist for this pull request

• I've read the Code of Conduct
• I've read the guidelines for contributing to this repository
• It builds for hardware native (make PROBE_HOST=native)
• It builds as BMDA (make PROBE_HOST=hosted)
• I've tested it to the best of my ability
• My commit messages provide a useful short description of what the commits do

Closing issues

fixes #1488

[dragonmux]

We've got one query, but other than that this looks good. Once the query's been answered and any required actions resulting taken, we'll happily approve this for merge.

[koendv]

OK. Let's set the aux serial write timeout to 25ms.
I prefer not to set the timeout too high, else if rtt is active and the usb aux serial is not being read, bmp might spend too much time busy-waiting.

[dragonmux]

Sounds good, and LGTM. Merging once the builds finish.