F4: merge all files from stevestrong master

[stevstrong]

Roger, this would bring the F4 part up to date with my current repo.
Yes, I know, there are a lot of changes, but in any case I intend to actively support this, as it will also flow back to my repo.

[rogerclarkmelbourne]

Thanks.

I will pull locally and merge

[rogerclarkmelbourne]

Thanks. I tested locally and it compiled OK.

I think I can safely delete the stevestrong_F4_merge branch in github

[ag88]

hi steve, roger,
how about merge the F3 core as well?
F303cc pill boards works fine with the codes in the current steve's core
https://robotdyn.com/stm32f303cct6-256-kb-flash-stm32-arm-cortexr-m4-mini-system-dev-board-3326a9dd-3c19-11e9-910a-901b0ebb3621.html
there are some things good about the F303CC that is not featured in another boards / mcu:
https://www.st.com/en/microcontrollers-microprocessors/stm32f303cc.html

1. 4x5 msps adc - 20 msps quad interleaved
2. comparators
3. op amps with programmable gain
4. not so important but capacitive sensing channels
5. dac !
   F303CC is my favorite 'analog' chip. most of other in stm32 series currently don't sport the same features. i think i contributed the rtc codes into steve's core for this. it is actually same as that for f4. still missing are the analog stuff, but the basic stuff works
   i think just F303CC alone can plot transistor characteristic curves over usb. it has all the basic ingredients, adc, dac, op amps and more for those

[rogerclarkmelbourne]

The F3 core in my repo was essentially just an archive to prevent it being lost forever.

But if Steve's version is functional, please feel free to merge it into my repo