docs: document PREFER_NATIVE_ARMHF and enrich arm64-compat-vdso entry

[iav]

Summary

Documents the new PREFER_NATIVE_ARMHF build switch landed in armbian/build#9820 and enriches the existing arm64-compat-vdso entry in the Extensions reference so the two cross-link sensibly.

• Developer-Guide_Build-Switches.md — new PREFER_NATIVE_ARMHF entry (default yes, opt-out no). Explains when it kicks in (arm64 host with CONFIG_COMPAT), what it changes (rootfs/chroot/post-install armhf steps run native instead of qemu, ~10× faster), and how to inspect the host kernel (zcat /proc/config.gz | grep ...) with sample output. Cross-links the arm64-compat-vdso extension and the arm64 Kconfig.
• Developer-Guide_Extensions-List.md — replaces the one-liner for arm64-compat-vdso with a description of the practical effect (native armhf userspace via CONFIG_COMPAT / CONFIG_COMPAT_VDSO / CONFIG_ARM64_32BIT_EL0), the build-time benefit, and the Apple-M-series limitation. Cross-links the switch.

Test plan

• MkDocs build (mkdocs build --strict) — no broken links / anchors.
• Visual check: Build-Switches PREFER_NATIVE_ARMHF → Extensions-List #arm64-compat-vdso round-trip resolves.

[coderabbitai]

Walkthrough

This pull request adds documentation for native armhf binary execution on arm64 build hosts. It introduces a new build switch PREFER_NATIVE_ARMHF with yes/no options, explains native execution when the host kernel has CONFIG_COMPAT (and notes CONFIG_COMPAT_VDSO), documents enabling that support via the arm64-compat-vdso extension, and provides a zcat /proc/config.gz check and a runtime limitation for platforms without 32-bit EL0 userspace support.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
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.
Description check	✅ Passed	The PR description clearly documents the addition of PREFER_NATIVE_ARMHF build switch documentation and enrichment of arm64-compat-vdso extension entry, with specific file changes and cross-references detailed.
Title check	✅ Passed	The title accurately describes the main changes: documentation for PREFER_NATIVE_ARMHF build switch and enriched arm64-compat-vdso extension entry.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@docs/Developer-Guide_Build-Switches.md`:
- Around line 348-351: Update the markdown code fence that wraps the example
lines "CONFIG_COMPAT=y" and "CONFIG_COMPAT_VDSO=y" to include a language tag
(e.g., use ```text) so the block is fenced as ```text ... ``` and satisfies
markdownlint MD040; locate the fenced block containing those two CONFIG_* lines
and prepend the opening backticks with the chosen tag.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 6b6d8181-63eb-447e-a823-d5da9e22d60c

📥 Commits

Reviewing files that changed from the base of the PR and between 2365684 and 7b4f000.

📒 Files selected for processing (2)

• docs/Developer-Guide_Build-Switches.md
• docs/Developer-Guide_Extensions-List.md

[EvilOlaf]

Why the ARMBIAN_ prefix of this switch?

[iav]

Agreed, dropping the prefix. Renaming to PREFER_NATIVE_ARMHF in armbian/build#9820, will update this PR.

[iav]

While I was doing this, I was thinking about large hosting machines and how best to determine whether they've included COMPAT in their kernels...

[EvilOlaf]

Just look for the config entry in /proc/config?

[iav]

Just look for the config entry in /proc/config?

Yes, as a possible method.
Am I wrong?

[EvilOlaf]

Unless there is a method having this present in kernel but be blocked on userspace level. No clue. Needs research

[iav]

Unless there is a method having this present in kernel but be blocked on userspace level. No clue. Needs research

Then just remove there lines 340-354?