BUGZILLA-1111371 Allow user to skip build phase.

If the user adds 'build' to the 'skip_stages' configuration variable,
then the build process will be skipped. Most of this change is to prevent
the temp directory from being clobbered on startup.

https://bugzilla.mozilla.org/show_bug.cgi?id=1111371
Document (barely) the new "build" skip_stages value.
Add "build" as a skip_stages value. Fix bug 1111371. Close #377.