Make start_dir available for steps

[Flamefire]

The config parameter start_dir is useful for finding the path to extracted contents. Example: In easybuilders/easybuild-easyconfigs#9604 %(builddir)s/KjellKod-g2log-d9a55a4a6154 could be simplified to %(start_dir)s if that was supported.

Maybe something better named would be good too but having the path to the extracted contents available as a template param would already help

[boegel]

started working on this in #3160, but don't have time to finish it right now, too much other stuff to look into...