Build-Switches: document KERNEL_DO_STUBBLE

[iav]

What

Adds a Build Switches entry for KERNEL_DO_STUBBLE in docs/Developer-Guide_Build-Switches.md (Advanced section, next to KERNEL_BTF).

The switch had no documentation anywhere — it was added to the build framework alongside the uefidt family / uefi-arm64-dt board but never described for users.

Why it matters

KERNEL_DO_STUBBLE=yes builds Canonical's stubble EFI stub and, during kernel packaging, runs ukify to assemble a Unified Kernel Image (.efi) that bundles the kernel, the stub, the device trees (auto-selected by finddtbs.py against a hardware-ID database via --devicetree-auto) and an SBAT section. The result is a single signed UKI that EFI-boots and picks the correct device tree for the running board automatically — the basis of generic UEFI-on-devicetree images.

It is enabled by default for the uefidt family and opt-in elsewhere; boards that boot through U-Boot do not use it.

Docs-only change.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 9e559124-01bb-49c5-9c58-7dc3bf9b9821

📥 Commits

Reviewing files that changed from the base of the PR and between 645b335 and 7a981a0.

📒 Files selected for processing (1)

• docs/Developer-Guide_Build-Switches.md

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Walkthrough

A new entry for the KERNEL_DO_STUBBLE build switch is added to the "Advanced" section of docs/Developer-Guide_Build-Switches.md. The entry documents the allowed values, describes that it enables building Canonical's stubble EFI stub and bundling it into a Unified Kernel Image using ukify with --devicetree-auto, explains the resulting single signed UKI behavior for automatic device tree selection, states it is enabled by default for the uefidt family and opt-in elsewhere, and lists ukify as a required build-host dependency.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main change: adding documentation for the KERNEL_DO_STUBBLE build switch in the Build-Switches documentation file.
Description check	✅ Passed	The description clearly explains what was added, why it matters, and the technical context of the KERNEL_DO_STUBBLE switch, directly relating to the changeset.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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