Something you may be interested in, if you keep thinking if it's rootdir or dirroot each time you use it. I've created moosh command that generates fake class moodle_config which is used only to provide auto completion and documentation for $CFG object. You can use moosh to generate your own or simply download one I've generated for Moodle 2.9 to get that feature, see here: http://moosh-online.com/#cfg-auto-completion .
I've tested it with netbeans and phpstorm, let me know if it does (or does not) work with your favourite IDEs.
cheers,
Tomek