Problem:
In some cases, as described in: https://github.com/zephyrproject-rtos/zephyr/issues/36948
shell backend sends characters to output before serial device
is ready for it. It results in observing additional characters
inserted on the shell input after device boot.
Solution:
Added waiting on DTR signal before sending anything to the output.
Fixes: #36948
Signed-off-by: Kamil Kasperczyk <kamil.kasperczyk@nordicsemi.no>
e9f238889b