fix(firmware): prevent LTO from eliminating the ISR vector table

[pfeerick]

Summary

• With LTO (-flto) enabled (currently used for X9D+2019 builds), the linker discards _isr_vector[] because no C code references it directly — the hardware reads it via the VTOR register. KEEP() in the linker script prevents gc-sections from removing it, but LTO operates before that stage and eliminates the symbol entirely, resulting in an empty .isr_vector section.
• This causes the firmware binary to have .fwversiondata (the version string) placed at the application start offset (BOOTLOADER_SIZE) instead of the ARM vector table, so isFirmwareStart() reads ASCII bytes instead of valid stack/ISR pointers and rejects the file as FC_ERROR.
• Fix: add used to the ISR_VECTOR macro in cortex_m_isr.h, matching the same pattern already applied to BOOTSTRAP and init_hooks() in system_init.c as part of the LTO work.
• Resolves regression introduced in feat(radio): add LTO option for complete firmware to reduce X9D+2019 #7255

Test plan

• Build X9D+2019 firmware with LTO and confirm .isr_vector section is non-empty in firmware.map
• Confirm valid firmware file is recognised in the bootloader (no longer shows FC_ERROR / "Invalid firmware")
• Confirm firmware boots correctly on hardware
• Verify non-LTO targets (X9D+, X9E, etc.) are unaffected

🤖 Generated with Claude Code

Summary by CodeRabbit

Bug Fixes

• Improved interrupt handler reliability by ensuring ISR vector entries are properly retained during compilation, preventing potential unintended removal by the linker.

[coderabbitai]

📝 Walkthrough

Walkthrough

The ISR vector table macro is updated to retain ISR entries in the final binary by adding the used attribute to the GCC section attribute, preventing the linker from discarding symbols that appear unreferenced.

Changes

ISR Vector Attribute Update

Layer / File(s)	Summary
ISR_VECTOR macro with used attribute radio/src/targets/common/arm/stm32/cortex_m_isr.h	ISR_VECTOR macro adds the used attribute alongside .isr_vector section placement to ensure ISR vector entries are retained by the linker.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and specifically describes the main fix: preventing LTO from eliminating the ISR vector table, which matches the core change in 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.
Description check	✅ Passed	The PR description comprehensively explains the problem, solution, and includes a test plan with verification status.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch pfeerick/fix-lto-isr-vector

Warning

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🔥 Problems

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[philmoz]

Tested ok on T14.

[craigywsm]

tested on my FrSky X9D Plus 2019 and it now boots