build: fix and improve FreeRTOS build system configuration

[heyonghau]

Summary

This PR fixes two issues with the FreeRTOS build system:

1. Remove redundant file collection in CMake: Previously, the build system added the FreeRTOS subdirectory and then searched for source files again to add them to compilation. This was redundant since add_subdirectory() already handles the source file compilation.

2. Fix USE_FREERTOS CMake option: Previously, running cmake -B build -DUSE_FREERTOS=ON would not properly enable FreeRTOS in the project. The LV_USE_OS macro was not being set correctly.

Testing

• Build with FreeRTOS: cmake -B build -DUSE_FREERTOS=ON && cmake --build build
• Build without FreeRTOS: cmake -B build && cmake --build build

Both scenarios now work as expected.

---

Summary by cubic

Fixes FreeRTOS build configuration and simplifies integration. USE_FREERTOS now correctly enables LVGL with FreeRTOS, and a new FREERTOS_HEAP option lets you pick the heap implementation.

• New Features

  • Added FREERTOS_HEAP CMake option to select heap_1–heap_5.

• Bug Fixes

  • USE_FREERTOS sets LV_USE_OS=LV_OS_FREERTOS via CMake.
  • lv_conf.h guards LV_USE_OS so CMake can override it.
  • Removed redundant FreeRTOS source collection; rely on add_subdirectory(FreeRTOS).

^{Written for commit 5ab4d27. Summary will update on new commits.}

[cubic-dev-ai]

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