Note: the custom UV option is only available when calling the operator
via a script. It's currently not exposed in the UI since it's intended
to be used by scripters
To test it:
bpy.ops.object.bake(type='UV', use_clear=True, uv_layer='MyNewUV')
Reviewers: campbellbarton
Differential Revision: https://developer.blender.org/D546