builder: Print some stats about the generated image at the end.

It will print stats at the end like this:

  Output: fedora-18.img (8.0G)
  Free space: 3.2G (39%)
builder: '@' (not '+') is used to install groups of packages in Fedora.
builder: Add comment about download /dev/stderr truncating redirection files.
builder: With -v option, dump out the log file.

Useful for debugging install errors.