Merge USB fixes

[maxgerhardt]

• Make USB core selectible
• rename some macros
• fix Windows USB-CDC by forcing a flush (better fix to be investigated)
• Update PlatformIO build script

[maxgerhardt]

Superseeded by https://github.com/CommunityGD32Cores/ArduinoCore-GD32/commit/a01c0afbcb4674adc35134b33adb725c3c6d4979