We've previously removed factory code (chromeos-factory-board) for follower projects on ToT/master, since follower projects should only have their code in factory branch, private overlay.

However maintaining follower boards by cloning from reference board overlay is still not the best approach.

We should move reference board implementations into public overlay, so follower boards can easily create a "ToT working toolkit" by inheriting right reference board.

In other words, currently in py/device/boards we only have
 LinuxBoard
 ChromeOSBoard
 AndroidBoard

In future we should move reference boards here, i.e.
 ReefBoard
 GruBoard
 OakBoard
 StragoBoard

So follower boards can do things easily like
 class ElmBoard(OakBoard):
   pass

Or even better if we can in future have some platform reference so follower projects can "auto select right board".