Move functionality of Makefile.shaders inside Makefile.griffin

To checkout shaders : make -f Makefile.griffin shaders-checkout
To checkout overlays: make -f Makefile.griffin overlays-checkout
To deploy overlays to PS3 media dir : make -f Makefile.griffin platform=ps3 shaders-deploy
To deploy overlays to PS3 cobra media dir : make -f Makefile.griffin platform=ps3-cobra shaders-deploy