GD32Cores for PIO build

[thinkyhead]

Hi! I'm just dropping a note to say that we over at MarlinFirmware maintain a keen interest in all the goings-on here in CommunityGD32Cores, with hope that we can support all boards regardless of MCU, while also not running afoul of STMicro restrictions on their USB library license.

Since we don't ship products but would only facilitate builds, license violation issues would fall on vendors who don't work out their licenses before shipping units. We can put up a strong warning as part of the build to make sure vendors are informed and that we are indemnified, which seems sufficient.

In the meantime I'm interested in providing a means for the Marlin build (via PlatformIO) to download and use CommunityGD32Cores for any boards that shipped (or are just floating around) with GD32 processors. We can collaborate here in this issue, if that works for you, or wherever you do your general brainstorming, such as Discord / Slack / etc.

[maxgerhardt]

Sounds great! I also dropped some notes earlier in MarlinFirmware/Marlin#21473 asking what makes sense that we do on our side. We have a discord server linked at https://github.com/CommunityGD32Cores/ArduinoCore-GD32#collaborating as communications channel for everyone, if you already have Discord we could just communicate there :)