Add support for building VirtualBox Vagrant images.
Abstract the build, package and upload to handle building
either type.

Fix an out-of-order execution issue regarding pkg(8):
- pkg(8) cannot be removed before subsequent reinvocations
- The PKG_CACHEDIR cannot be cleaned after the repo*.sqlite
  has been removed
- pkg(8) cannot be removed as a precursor to any of the other
  steps involved here