initial support for xiao_c6

[fdlamotte]

Just following in @recrof steps ;)

[ripplebiz]

I've had to remove the C6 stuff, because it seems to be corrupting people's platformio caches, and is forcing complete re-installs :-( Until this can be figured out, will need to remain experimental for now.

[IoTThinks]

@liamcottle @recrof
I have fixed the bootloop. #2570
In Windows and Git Bash, I find it fine to compile Xiao C6 and other boards fine without any conflicts or cache corruptions.

Hope you can try the Xiao C6 again and add to Web Flasher.
Xiao C6 is cheap and very power efficient.