boot: boot_serial: add optional mcumgr parameters command

[JPHutchins]

Summary

Adds an optional, Kconfig-gated mcumgr MCUmgr parameters command to the
serial-recovery SMP server (OS management group 0, command ID 6,
read-only), mirroring the command provided by Zephyr's SMP server.

When CONFIG_BOOT_MGMT_MCUMGR_PARAMS is enabled the server answers with the
CBOR map {"buf_size": <CONFIG_BOOT_SERIAL_MAX_RECEIVE_SIZE>, "buf_count": 1},
allowing SMP clients (e.g. smpclient)
to auto-negotiate serial fragmentation instead of hardcoding buffer sizes.

buf_count is 1 because serial recovery reassembles one command at a time
into a single buffer.

Why the 128-byte line-length requirement

mcumgr parameters do not carry the transport line length (MTU); SMP serial
clients assume 128-byte fragments and derive line_buffers = buf_size // 128.
To keep the advertised buf_size consistent, the option
depends on BOOT_MAX_LINE_INPUT_LEN = 128 — setting a different line length
makes Kconfig refuse the option.

Compatibility

Identical wire contract to upstream Zephyr's os_mgmt_mcumgr_params — same
group/id, read-only, same buf_size/buf_count keys and zcbor_uint32_put
encoding — so existing SMP clients read it unchanged.

ROM / RAM cost

Measured on nrf52840dk/nrf52840 with serial_recovery.conf, with vs.
without the option: +116 B flash, +0 B RAM.

Test coverage

Added CONFIG_BOOT_MGMT_MCUMGR_PARAMS=y to the serial_recovery_all_options
twister scenario, so the feature is build-covered on every PR via the existing
Zephyr twister CI.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds an optional, Kconfig-gated mcumgr "MCUmgr parameters" command (OS group, command ID 6) to MCUboot's serial-recovery SMP server. When enabled, it replies with a CBOR map {buf_size, buf_count} so that SMP clients can auto-negotiate serial fragmentation, mirroring Zephyr's os_mgmt_mcumgr_params command. The option is gated on BOOT_MAX_LINE_INPUT_LEN = 128 to keep the advertised buffer size consistent with the 128-byte fragment size assumed by SMP serial clients.

Changes:

• New bs_mcumgr_params handler in boot_serial.c and a new NMGR_ID_MCUMGR_PARAMS (6) dispatch case, all guarded by MCUBOOT_BOOT_MGMT_MCUMGR_PARAMS.
• New Kconfig option BOOT_MGMT_MCUMGR_PARAMS (with the BOOT_MAX_LINE_INPUT_LEN = 128 dependency) and corresponding mcuboot_config.h macro mapping.
• Sample/twister scenario, release note, and serial_recovery.md updates documenting the new optional command.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
boot/boot_serial/src/boot_serial.c	Implements bs_mcumgr_params and wires it into the OS-group dispatch switch under the new ifdef.
boot/boot_serial/src/boot_serial_priv.h	Adds the NMGR_ID_MCUMGR_PARAMS (6) command-ID constant.
boot/zephyr/Kconfig.serial_recovery	Adds the BOOT_MGMT_MCUMGR_PARAMS Kconfig option, depending on BOOT_MAX_LINE_INPUT_LEN = 128.
boot/zephyr/include/mcuboot_config/mcuboot_config.h	Maps the Kconfig symbol to the port-agnostic MCUBOOT_BOOT_MGMT_MCUMGR_PARAMS macro.
boot/zephyr/sample.yaml	Enables the new option in the serial_recovery_all_options twister scenario for build coverage.
docs/serial_recovery.md	Documents the new command in the supported-MCUmgr-commands list.
docs/release-notes.d/serial-recovery-mcumgr-params.md	Adds a release note for the new option.

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[JPHutchins]

Superseded by the upstream PR mcu-tools#2746 (this was an internal fork PR opened by mistake).