alternative source for atdf files

[khimaros]

https://github.com/avr-rust/avr-mcu/tree/master/packs

would it make sense to deduplicate?

[Rahix]

I quite like having the files vendored here. It's simpler than adding a submodule and having to keeping track of that, we're free in updating/adding/removing individual files, and I would argue the files aren't big enough to warrant caring too much about this.

[khimaros]

in that case, maybe worth documenting the source i mentioned?

navigating the atmel site is a bit frustrating and i failed to locate the atdf for at90usb1286

[Rahix]

Done :)