Arduino_core_STM32 support (improved)

[dirkju]

Adding Arduino_core_STM32 support

Three previous attempts of adding STM32 support were abandoned (#1422, #1437, #1486).
@tekka007 s branch also served as inspiration. Thanks to everyone of you.

I'm asking you to please consider my attempt :-) @tekka007 @mfalkvidd

Risks

The change is rather local in hal/STM32 and should not cause problems with other architectures.
I've kept the existing legacy hal/STM32F1 unchanged and the respective constant is checked in MySensors.h before my STM32 addition. This should maximize backward-compatibility for STM32F1 (blue pill) legacy users.

Implemented in this version

• Serial communication (USB CDC and Hardware UART)
• SPI interface for radios
• EEPROM emulation using Flash memory
• Watchdog support (requires explicit initialization)
• System reboot
• Random number generation (using internal temperature sensor)
• Unique device ID (96-bit STM32 UID)
• CPU voltage reading (via VREFINT)
• CPU temperature reading (via internal sensor)
• CPU frequency reporting
• Critical section (interrupt disable/restore)
• RAM routing table support

Planned

• Low-power sleep modes (STOP, STANDBY)
• RTC-based timekeeping
• Interrupt-based wake from sleep
• Free memory reporting (heap analysis)

Build

I developed under VS Code + Platformio on Windows. I test-compiled examples under the Arduino IDE (also Windows).

Testing

I've tested mostly on a generic black pill STM32F401CE (gateway, node using Ra-01 LoRa and nRF24L01+).
I encourage to test on a broader variety of boards.

Final note

The code has mostly been generated using AI. I asked for a comprehensive review of the previously mentioned three abandoned STM32 PRs and a comparison between building on STM32duino vs the original STM32Cube HAL. Both are feasible but the simplicity of the STM32duino solution stood out.