There's no need to wrap the attributes in a platform check

This would just cause the recipe to insert blank values during
execution.  A better solution would be a platform check in the recipe
Make repositories an attribute

maintains the default, but allows users to decide what repositories to
use
Update readme and metadata description
Change repositories to components.
Merge pull request #4 from tas50/COOK-874

Cook 874: Allow configuring specific repositories to load via an attribute