reworked doom libretro for new build location / system
move some functions out of helpers that are not used by multiple scripts
moved rps_availFreeDiskSpace to retropiesetup.sh
removed some unused functions from helpers.sh
renamed rps_checkNeededPackages to checkNeededPackages as it is used by modules and setup
we need to switch to using libretrocores rather tham emulatorcores to match with scriptmodule folder names. Seems a more logical name also.