When the west rtt command is executed, two things happen:
- A GDB server is run.
- A telnet client (via sockets) is opened to connect to that server.

Previously, that telnet client was operating in the line mode in both
directions. That mode is perfectly fine for just presenting the captured
content (RTT data) to the stdout; however, it is not suitable for
interacting with the device via the shell subsystem. None of the
control commands, such as tab and arrow keys, were correctly passed down
to the device.

To solve that the telnet client had to be swithced into the character
mode. For that two separate implementations were needed, one for the
Unix and one for Windows.

Signed-off-by: Marko Sagadin <[email protected]>

This commit introduces --device-rtt flag in the Pytest Twister harness,
allowing RTT as a communication protocol between the host PC and the
DUT.

To demonstrate this, a new build configuration was added to the
pytest/shell sample. By following the newly added instructions in the
scripts/pylib/pytest-twister-harness/README.rst, users can first build
the pytest/shell sample with the RTT shell and then run the pytest
with specific flags to run the tests via the RTT.

Two additional notes related to the changes:
- The way the pty process was terminated had to be changed. west rtt
  command spawns two subprocesses, and the previous implementation only
  terminated a single one. The already provided terminate_process
  function was used to terminate all child processes that pty process
  started.
- Previously, exec_command sent out a command string, followed by two
  newline characters. The first one was meant to execute the command on
  the device, the second one was meant to produce the next prompt, which
  would signal that execution of the command finished. In some
  instances, when RTT was used, the device ignored the second newline,
  and thus the prompt was not emitted, bringing the function to a halt,
  until the timeout expired. This was noticed with the `kernel version`
  command, the one that is used in one of the tests in pytest/shell.
  Weirdly enough, that was the only instance of the "newline ignoring"
  that could be found. Testing `kernel uptime`, for example, worked
  fine. The workaround was to send a command string with a single
  newline, wait for the command print and then send the second newline.
  It was confirmed that this approach works both for the UART and RTT.

Signed-off-by: Marko Sagadin <[email protected]>