Full F30x support

[maxgerhardt]

We currently only support a hand-picked, inherited GD32F303CC board definition.

We need to work more on the genpinmap script to successfully spew out all F30x Arduino variant files when fed the datasheet PDFs.

Treating a e.g. F303CG chip as a F303CC chip fails even when adapting some of the macros as seen in #92.