Allow time for the shell to successfully echo the reboot command input
before the reboot abruptly terminates it. n This can help external
systems that interact with the shell and require the reboot command's
echo to successfully complete to synchronise with the device.
Fixes: https://github.com/zephyrproject-rtos/zephyr/issues/35325
Signed-off-by: Nick Ward <nick.ward@setec.com.au>