[action] [PR:4602] [sonic-installer] Add dedicated SONiC-BMC U-Boot bootloader

[mssonicbld]

Address issue - sonic-net/sonic-utilities#4548

What I did

Added a dedicated BmcUbootBootloader so sonic-installer manages SONiC-BMC boot state correctly. SONiC-BMC platforms (ASPEED AST2700) run U-Boot with a two-slot environment, which the generic UbootBootloader does not model — on a BMC it previously fell through to that generic handler and operated on the wrong environment variables.

Specifically:

• New sonic_installer/bootloader/bmc_uboot.py (BmcUbootBootloader, NAME = 'bmc-uboot'), selected via utilities_common.chassis.is_bmc().
• Registered first in BOOTLOADERS so a stray /host/grub/grub.cfg cannot preempt it, and guarded UbootBootloader.detect() to exclude BMC.
• Full two-slot lifecycle: list, set-default, set-next-boot, install, remove, cleanup, verify-next-image, set-fips/get-fips, binary-version, plus a BMC-specific verify_image_platform (fails closed).
• Unit tests in tests/installer_bootloader_bmc_uboot_test.py (12 tests).

No change to sonic_installer/main.py — the work is confined to the bootloader plugin and its test.

How I did it

BmcUbootBootloader subclasses OnieInstallerBootloader and models the ASPEED AST2700 two-slot U-Boot environment:

Concept	Slot 1	Slot 2
version var	sonic_version_1	sonic_version_2
boot command	run sonic_image_1	run sonic_image_2
kernel args	linuxargs	linuxargs_old

Empty slots are written/read as the marker None (also accepts NONE/empty, case-insensitively).

• Boot selection (Aboot-style contract): set-default writes the persistent boot_next and clears the one-shot boot_once; set-next-boot writes only boot_once. get_next_image() resolves boot_once first (it wins, matching U-Boot bootcmd), then boot_next, and surfaces an empty/unrecognized selector as a raw string so a broken boot state stays visible rather than being masked.
• Install: install_image() runs the image's own installer (bash <image>, which performs the slot rotation and sets boot_next) and then clears any stale boot_once that would otherwise shadow the new default.
• Remove: remove_image() repoints boot_once/boot_next to the surviving slot before clearing the removed slot's version + auxiliary vars and deleting its rootfs; it refuses to remove the only populated slot.
• FIPS: set-fips/get-fips rewrite the sonic_fips= token in the target slot's linuxargs/linuxargs_old only (per-slot isolation).
• Platform check: verify_image_platform() mirrors the GRUB approach — extracts installer/platforms_asic from the image and matches the running device_info.get_platform() with grep -Fxq (whole-line). It fails closed: compatible only when grep matches, so a malformed/manifest-less payload is rejected rather than silently accepted.

How to verify it

On a SONiC-BMC (ASPEED AST2700) board, the dedicated bootloader is auto-selected and the full command surface was exercised end-to-end, cross-checking the U-Boot environment (fw_printenv) and, across real reboots, /proc/cmdline:

• sonic-installer list / verify-next-image reflect the two-slot state.
• set-default <img> sets boot_next and clears boot_once; set-next-boot <img> sets a one-shot boot_once that U-Boot consumes on the next reboot and then reverts to boot_next.
• install <bmc.bin> rotates the new image into a slot and boots it after reboot.
• remove / cleanup clear the slot's vars, repoint the boot selectors, and delete the rootfs.
• set-fips/get-fips toggle sonic_fips= in the correct per-slot args.
• A wrong-platform (switch) image is rejected by verify_image_platform; --skip-platform-check overrides it.

Signed-off-by: Sonic Build Admin sonicbld@microsoft.com

Previous command output (if the output of a command-line utility has changed)

New command output (if the output of a command-line utility has changed)

[mssonicbld]

Original PR: sonic-net/sonic-utilities#4602

[mssonicbld]

/azp run

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