move some duplicated functions to helpers.shinc and remove duplicated startup code from retropie_setup.sh. retropie set up now gets it's
initial environment from retropie_packages.sh so there is no duplication of functionality. However it might be even cleaner to have a core module that contains this stuff and perhaps another module for packages, so the root shell scripts become simplified.
we can check for required packages just once in retropie_packages.sh
use script name in usage info
Merge pull request #379 from joolswills/de-duplication

move some duplicated functions to helpers.shinc and remove duplicated st...