README board selection instructions are stale

[airadier]

Summary

The README says board selection is done via add_compile_definitions(...) in main/CMakeLists.txt, but that define is not present in the repository.

Impact

A new contributor can follow the documented setup and still not understand how board selection is actually supposed to happen.

Evidence

• Documented in README.md:22-29
• No matching definition in CMakeLists.txt files
• Actual selection currently appears to be implicit in main/board.h

Expected

README should describe the real mechanism for selecting BOARD_DEVKIT vs default M5Stick target, or the build system should be updated to match the docs.