arduino: new package

This commit adds the graphical Arduino IDE, built from the arduino-core
expression.

Also:
  - Add libusb to RPATH for avrdude. Without this, avrdude is broken.
    Possible TODO: replace bundled avrdude with the one from nixpkgs.
  - Use "cp -r ./build/linux/work/*", so example sketeches, the HTML
    reference documentation and the 'arduino' shell script that starts the
    IDE gets included.
  - Make meta.description slightly more generic, to cover both GUI/core
    variants.