Add option '--skip-to-postinstall' to build command.

This will skip the installation and go streight to postinstall. It is
usefully for testing you post-install scripts without running throug
the whole, time-consuming install process.
Add documentation for option '--skip-to-postinstall'.
Do not try to start the box if it is already running.
Merge pull request #896 from htgoebel/shortcircuit-postinstall

Shortcircuit postinstall