Remove example data and add repo setup support

- Example data should be in its own branch, following our own advice to
  keep master as bare as possible.
- Since `target` is overlayed _after_ software installation, this made
  for an awkward bug where if you added a repository in the overlay, it
  wouldn't be applied before we try to install it. Solve by adding
  another build step in which we copy all the repos from the
  `software/repos` directory into the proper place _prior_ to the
  software install step.