app: Enable UART application log by default

[MarkusLassila]

• Application logs are by default going to UART1, when the AT#XLOG=1 is given.
• RTT application logs are disabled by default, but can be enabled in prj.conf if required.
• UART1 is enabled by default, but suspended in startup.
• overlay-trace-backend-uart.conf only includes AT#XTRACE.
• overlay-trace-backend-uart.overlay is removed.

Jira: SM-296, SM-332

[Copilot]

Pull request overview

This PR switches the Serial Modem application’s default logging backend from SEGGER RTT to UART1 (VCOM1 on nRF9151 DK), with the UART kept suspended until logs are enabled at runtime via AT#XLOG=1. It also separates UART logging from the modem trace backend so that AT#XLOG is always present, while AT#XTRACE remains opt-in via an overlay.

Changes:

• Default app logging moved to UART1 with CONFIG_LOG_BACKEND_UART_AUTOSTART=n and runtime enable/disable via AT#XLOG.
• Modem trace backend (AT#XTRACE) kept behind overlay-trace-backend-uart.conf; the devicetree overlay for selecting/configuring UART1 was removed and equivalent settings were moved into board overlays.
• Documentation and migration notes updated to reflect the new default logging behavior and how to re-enable RTT.

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
doc/releases/migration_notes_v2.0.0.rst	Adds a required migration note about the default log backend moving to UART and runtime enable via AT#XLOG=1.
doc/releases/migration_notes_ncs_slm_v3.1.x.rst	Adds the same migration note for users migrating from NCS SLM v3.1.x.
doc/links.txt	Adds a new link target for nRF9151 anomaly 36 (used by the updated logging docs).
doc/app/sm_logging.rst	Updates logging documentation: UART1 default, RTT re-enable instructions, and adds anomaly note.
doc/app/at_trace.rst	Updates prerequisites: AT#XLOG always available; AT#XTRACE requires the Kconfig overlay only.
app/src/sm_trace_backend_uart.c	Refactors UART device naming/handling; removes XLOG handling from this file; adds startup suspend via SYS_INIT.
app/src/sm_log_uart.c	New module implementing AT#XLOG and suspending/resuming the console UART to control UART log backend at runtime.
app/sample.yaml	Removes the deleted trace devicetree overlay from CI build args.
app/prj.conf	Switches defaults to UART logging (autostart off) and disables RTT by default.
app/overlay-trace-backend-uart.overlay	Removed; trace backend no longer relies on this overlay for routing/configuring zephyr,console.
app/overlay-trace-backend-uart.conf	Now only enables the UART modem trace backend (AT#XTRACE) and forces modem trace level off at boot.
app/CMakeLists.txt	Adds the new sm_log_uart.c source file to the build.
app/boards/thingy91x_nrf9151_ns.overlay	Routes zephyr,console to uart1 and configures UART1 at 1,000,000 baud.
app/boards/nrf9151dk_nrf9151_ns.overlay	Routes zephyr,console to uart1 and configures UART1 at 1,000,000 baud.

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[MarkusLassila]

This is intended. If customer changes to RTT-traces, they do not need to disable the UART from the DTS, although they are free to do so.

[divipillai]

Suggested change

	`nRF9151 anomaly 36`_ causes debug port to be locked when the application reaches low power state (<3 uA current consumption).
	This takes place when DTR is deasserted and RTT client, such as J-Link RTT Viewer, is not connected.
	Since the RTT backend relies on the debug port, the RTT client must be connected before the application enters low power state to avoid this issue.
	`nRF9151 anomaly 36`_ locks the debug port when the application reaches a low power state (<3 uA current consumption).
	This takes place when DTR is deasserted and the RTT client, such as J-Link RTT Viewer, is not connected.
	Since the RTT backend relies on the debug port, the RTT client must be connected before the application enters a low power state to avoid this issue.