Support for STM32F4

[ipsod]

A lot of the new 8-axis printer boards are running STM32F4 chips.

I understand if there isn't time, I just wanted to put this on the map.

TeensyStep supports these boards - it may provide a good reference.

[gin66]

good proposal. PR‘s are welcome

[ajay01994]

Hi ,
I see the teensy step doesnt support teensy 4.0 and above, do you have any reference implementation