add support for H503RB (generic and Nucleo)

[VIPQualityPost]

Adding the "new" STM32H503 with nucleo (these are already supported in stm32duino).

[VIPQualityPost]

@valeros same for this one 😄

[valeros]

Hi @VIPQualityPost, thanks for the PR. The latest stable OpenOCD doesn’t support the H5 family, and we don’t use any OpenOCD forks. So, basically, you’ll have to drop st-link and cmsis-dap from the list of upload protocols. That means the board will need to be programmed either through mass storage (which I think the latest ST-Links support) or using an external J-Link debug probe, but that’s going to make the developer experience pretty inconvenient.

[multiplemonomials]

btw, NUCLEO_H503RB is supported in mbed-ce, so you can add mbed-ce to the frameworks if #863 merges first.

[multiplemonomials]

fwiw, I think pyocd actually does support these boards now, even though openocd and st-link still do not. So you might want to try adding cmsis-dap as an upload protocol. Though if platformio is still using an old pyocd version that may not be the case.

[VIPQualityPost]

I wasn't really paying attention to this PR anymore because of this comment: #853 (comment)

Did the 'new' method for supporting generic hardware get implemented yet? ( @valeros )
I didn't plan to do any changes here until the plan is set for the new support scheme.