Lubuntu 13 using pre-installed edX image

Revised version of #2, with renamed branch. See comments there. 

**UNTESTED!** *All my SD cards are in use. These changes should work, but it would be great if someone could test them before they get merged. Original changelog follows.*

execute sd now downloads a rootpart with edX pre-installed.
Steps that previously failed if they'd already been done now check before running