fix: derive SWSERIALPOWER from hardware capabilities

[3djc]

automagicaly define SWSERIALPOWER, so that lua serialSetPower() is available everywhere a compatible hardware exists

Fixes #7389

Summary by CodeRabbit

• Chores
  • Modified serial power configuration handling to determine settings based on hardware GPIO availability rather than build configuration.

[coderabbitai]

📝 Walkthrough

Walkthrough

Serial power configuration for auxiliary serial ports is relocated from CMake-based build flags tied to PCB revisions into automatic hardware-driven detection via GPIO macro availability. The header file now defines SWSERIALPOWER when appropriate GPIO definitions exist, and the CMake build file removes all related flag assignments and conditional compilation.

Changes

Serial Power Configuration Refactoring

Layer / File(s)	Summary
Hardware-driven SWSERIALPOWER definition radio/src/hal/serial_port.h	A preprocessor conditional defines SWSERIALPOWER automatically when AUX_SERIAL_PWR_GPIO or AUX2_SERIAL_PWR_GPIO is defined, replacing the previous build-time configuration approach.
Remove CMake-based SWSERIALPOWER configuration radio/src/targets/horus/CMakeLists.txt	Removed two SWSERIALPOWER flag assignments from PCB revision branches and the conditional block that added the -DSWSERIALPOWER compile definition, completing the transition to header-based configuration.

🎯 2 (Simple) | ⏱️ ~8 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description explains the purpose (auto-define SWSERIALPOWER for Lua serialSetPower availability) and references the linked issue #7389.
Linked Issues check	✅ Passed	The changes enable serialSetPower() availability on compatible hardware by automatically defining SWSERIALPOWER based on GPIO presence, directly addressing issue #7389's requirement for parity between Mk2 and Mk3.
Out of Scope Changes check	✅ Passed	All changes are scoped to the SWSERIALPOWER definition mechanism: header macro addition and CMake build flag removal—both directly supporting the objective.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Title check	✅ Passed	The title directly reflects the main change: automatically deriving SWSERIALPOWER from hardware capabilities instead of manual CMake configuration.

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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 3djc/serialpower

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