lualoader: Move carousel storage out into config

Carousel storage doesn't need to happen in the menu module, and indeed
storing it there introduces a circular reference between drawer and menu
that only works because of global pollution in loader.lua.

Carousel choices generally map to config entries anyways, making it as good
of place as any to store these. Move {get,set}CarouselIndex functionality
out into config so that drawer and menu may both use it. If we had more
carousel functionality, it might make sense to create a carousel module, but
this is not the case.