Make it possible to use the plugin for all known Atreus variants: the post-2016
PCB with A* and the FalbaTech handwired one with a Teensy (these two were the
only supported ones until now), the pre-2016 PCB with A*, and the legacy Teensy2
variant.
Fixes#430.
Signed-off-by: Gergely Nagy <algernon@keyboard.io>