app: PIPE interface to UART

[MarkusLassila]

Jira: SLM-134

Implement PIPE interface to Serial Modem UART to be able to use the same UART implementation for both pure AT-commands and for CMUXed AT-commands and PPP.

[Copilot]

Pull Request Overview

This PR implements a PIPE interface to the Serial Modem UART to enable unified UART implementation for both pure AT-commands and CMUXed AT-commands with PPP. The changes streamline the UART backend architecture by consolidating functionality and removing the SLM-specific backend dependency.

• Replaces backend-specific UART implementations with a unified pipe interface
• Refactors AT command processing to support CMUX switching with proper state management
• Standardizes error message formatting across initialization functions

Reviewed Changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
doc/app/sm_description.rst	Updates documentation for PPP configuration files
app/src/sm_uart_handler.h	Adds pipe interface declarations and transmit callback typedef
app/src/sm_uart_handler.c	Implements unified UART pipe interface with CMUX support
app/src/sm_ppp.c	Switches from SLM-specific to generic UART backend
app/src/sm_defines.h	Adds CMUX command return code constant
app/src/sm_cmux.h	Adds uninit function declaration
app/src/sm_cmux.c	Refactors CMUX to use new pipe interface and removes SLM dependency
app/src/sm_at_host.h	Updates AT receive function signature
app/src/sm_at_host.c	Removes backend abstraction and integrates direct UART calls
app/src/sm_at_commands.c	Standardizes error message formatting
app/overlay-ppp.conf	Removes SLM backend configuration
app/overlay-ppp-without-cmux.conf	Adds generic UART backend configuration
app/overlay-ppp-cmux-linux.conf	Updates buffer sizing and removes SLM configuration
app/overlay-cmux.conf	Removes SLM backend and buffer configuration

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[Copilot]

Pull Request Overview

Copilot reviewed 14 out of 14 changed files in this pull request and generated 4 comments.

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[trantanen]

Add Jira: SLM-xx into PR description.

[trantanen]

@moksanen, @jvaliharju
When this goes in hopefully at some point next week, a new Kconfig overlay has to be taken into use for AT+PPP without CMUX config: EXTRA_CONF_FILE="overlay-ppp-without-cmux.conf"