shell: uart: Add waiting on DTR signal before sending data

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>