STM32 support available now in development branch

[dirkju]

Dear Community,

Happy holidays everyone! Just wanted to let you know that STM32 (bluepill and blackpill) support has been added to the development branch.

It's based on latest STM32Duino platform and unlocks usage of the STM32 architecture beyond the legacy STM32F1 (older hardware) that MySensors had over the last years. BlackPill hardware is more recent and improved and with the STM32Duino base, we have a future-proof dependency.

Enjoy!
Many thanks to tekka and fallberg for support and code reviews!

/Dirk