Generic OpenBLT bootloader backed by Arduino_Core_STM32

[rhapsodyv]

I created a generic bootloader for any STM32 mcu supported by this framework and OpenBLT.

Project link: https://github.com/rhapsodyv/OpenBLT-STM32

I took Open BLT and used Arduino_Core_STM32 as HAL. So, any supported stm32 mcu should work.

Right now, it only support SD (spi) firmware upload.

It's just at beginning. But it may be useful for a lot of Arduino_Core_STM32 users.

Soon I will add more features and more upload methods support.