Contribution description

Drop the requirement of having only one writer and one reader, as the name of the ring-buffer does not indicate any limitation on the thread-safety. The two-threads-one-buffer kind of ring buffer can be reintroduced with a different name.

Testing procedure

On e.g. STM32 or SAM0 boards, have two threads continuously produce stdio output. The thread with higher priority should sleep for random intervals.

Expected output:

• The higher priority threads interrupts the lower priority thread and stdio output gets mixed up. But no characters are lost or printed more than once

Actual output:

• Occasionally, chars get lost or printed more than once

Issues/PRs references

Fixes #9882

Split out of #15103