We only ever do one of two things with the global path options:
1) Reference the option in the default of another option so that the
value can be interpolated it
2) Use the value of the option to build a path
Add helper methods for both these cases - e.g. basedir_def() for the
former case and basedir_rel() for the latter case. This makes it much
more obvious how and where these options are used.
Change-Id: I7fd94a329fe911761d02d94e5381e950c6668d56
1428956919