bcachefs: use UUID for rootfs in kernel cmd line

[jackwasey]

As of September 2025, CachyOS default install with a single-drive bcachefs filesystem for root works. However, with more than one disk in the bcachefs filesystem, sdboot-manage uses the colon-delimited list of devices for root= in the kernel command line, which fails. This patch will find the bcachefs filesystem UUID, and use that for root= instead.

[Copilot]

Pull Request Overview

This PR adds support for using UUID-based root filesystem specification in kernel command lines for multi-disk bcachefs filesystems. Currently, sdboot-manage fails when bcachefs spans multiple devices because it uses a colon-delimited device list that the kernel cannot parse.

• Adds bcachefs case handling to extract filesystem UUID from the first member device
• Implements validation to confirm the UUID matches the correct bcachefs filesystem
• Updates kernel command line to use root=UUID=<uuid> format instead of device paths

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