RAK3401: Missing I2C/GPS pin definitions & nRF52840 BSEC linker failure

[martenumberto]

While setting up the RAK3401 target, two critical configuration and build issues were identified:

1. Missing / Incomplete Board Pin Mappings:
   • The RAK3401 variant definition lacks key standard pin assignments for onboard/WisBlock Slot I2C communication (PIN_BOARD_SDA=13, PIN_BOARD_SCL=14) as well as dedicated GPS UART pins.
   • Without these explicit definitions, attached I2C sensors (e.g., BME series) and GPS modules fail to initialize or require manual overrides in user code.
2. nRF52840 BSEC Library Linker Error:
   • When compiling MeshCore with ENV_INCLUDE_BME680_BSEC=1 for nRF52840 targets, the build fails during the linking stage.
   • Root Cause: Bosch provides the pre-compiled BSEC library with a soft-float ABI, whereas the nRF52840 toolchain compiles with hardware floating-point support (-mfloat-abi=hard). This ABI mismatch causes GCC/ld to throw a fatal architecture/ABI conflict error.

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Proposed Solution: Pull Request #2716

Pull Request #2716 addresses both issues cleanly:

1. Board Definition Update (rak3401):

   • Corrects and completes the pin mapping for PIN_BOARD_SDA, PIN_BOARD_SCL, and GPS TX/RX interfaces according to the RAK WisBlock schematic.

2. Automated BSEC Patch Script (fix_bsec_lib.py):

   • Adds a extra script / patch helper in PlatformIO that patch-sets/handles the ABI flag for the BSEC static library specifically for nRF52840 build steps, allowing -mfloat-abi=hard builds to link smoothly without throwing architecture mismatch errors.

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Related Links / PR

• Pull Request: Added several Flags to fix some issues with RAK3401 #2716