RFC: Get rid of shell UART device selection in Kconfig

[de-nordic]

Introduction

The selection of UART device for usage with shell seems redundant when it is possible to use DTS overlay to do so.
The configuration item is UART_SHELL_ON_DEV_NAME.

Problem description

Possibility of selecting UART device via Kconfig causes additional problem as it adds additional level, to existing support for DTS overlays.
As a result it is possible to override board dts on two levels which is redundant. This may also cause problem when user decides to change UART assignment via DTS overlay, being unaware that configuration will override it.

Proposed change

Remove possibility to select UART device assigned to shell via Kconfig, by removing UART_SHELL_ON_DEV_NAME configuration
item, and rely on DTS overlays overriding the zephyr,shell-uart alias.

Dependencies

All project using the UART_SHELL_ON_DEV_NAME would need to include DTS overlay instead.

Concerns and Unresolved Questions

N/A yet.

Alternatives

Current state. Removal of zephyr,shell-uart support from DTS does not seem to be good option.

[pabigot]

How would this work if the shell UART is to be on a USB CDC-ACM device, for which there are no devicetree nodes?

[pabigot]

The underlying issue was reported in #27597 for CONFIG_UART_CONSOLE_ON_DEV_NAME. This seems a fundamental weakness of /chosen/zephyr,foo, but I don't see how to eliminate it. That the devicetree property merely sets the initial value of the Kconfig symbol is a bit subtle.

[de-nordic]

The issue does not seem to be resolvable currently and does not seem to be important.