Merge patch series "bootstd: New bootmeth for RAUC A/B systems"

Martin Schwan <m.schwan@phytec.de> says:

This series implements a new bootmeth for RAUC A/B systems. RAUC (Robust
Auto Update Controller) is a lightweight update client, providing "Safe
and Secure OTA Updates for Embedded Linux". See the following links for
more information about RAUC:

  https://rauc.io/
  https://rauc.readthedocs.io/en/latest/

PHYTEC uses RAUC in its Yocto based distribution "Ampliphy" as the
default way of updating embedded devices based on PHYTEC hardware. So
far, the logic selecting the correct partitions and files to boot was
being implemented in the U-Boot environment. While this is a
straightforward way to do it, adding and supporting new platforms became
somewhat tedious and is platform-specific. The introduction of U-Boot's
"Standard Boot" provided a convincing alternative, promising a simpler
and more portable way of booting, even for RAUC systems. This led me to
implement a new bootmeth supporting RAUC A/B systems. Note, that this
new bootmeth is not proprietary to PHYTEC products and is designed to
work on other hardware with a RAUC A/B system, too.

The bootmeth currently only supports symmetric A/B partitioning layouts.
E.g. A/rescue is not (yet) supported. The partition indexes and default
slot tries can be specified via configuration options.

For now, the bootmeth_rauc uses a similar approach for loading the
Kernel and device tree as the bootmeth_script, in that it requires a FIT
containing a U-Boot script loading the desired distro. It could be
possible to support booting without a script and load the Kernel and DT
directly with this bootmeth, but I found the script method to be very
flexible for now, in letting the distro decide what to load.

The bootmeth_rauc was tested on a phyBOARD-Pollux i.MX8M Plus [1] with
BSP-Yocto-Ampliphy-i.MX8MP-PD24.1.2 [2].

Supported boot devices are currently only MMC devices, but it should be
possible to add SPI flashes in the future.

To test this patch stack with PHYTEC's phyBOARD-Pollux i.MX8M Plus
board, you need to adjust the boot files to include the boot.scr.uimg
containing the distro's boot script and set "optargs" to "${raucargs}"
in it. Also disable any legacyboot in the U-Boot environment and simply
boot with Standard Boot:

  bootmeth order rauc
  bootflow scan -lb

[1]: https://www.phytec.eu/en/produkte/single-board-computer/phyboard-pollux/
[2]: https://download.phytec.de/Software/Linux/BSP-Yocto-i.MX8MP/BSP-Yocto-Ampliphy-i.MX8MP-PD24.1.2/

Link: https://lore.kernel.org/r/20250604-wip-bootmeth-rauc-v3-0-f9fad913c57e@phytec.de
[trini: Don't enable by default]

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