Handle dynamic layers gracefully

[haadr]

When you disable a layer for a certain product the path of the layer will be masked using BBMASK. However, the layer will still be in the BBFILE_COLLECTIONS at parse time. This means that dynamic layers, added by layer.conf files using BBFILES_DYNAMIC, are still added even though we've marked them as not-in-use for a product in whisk.yaml. This may generate warnings about not being able to find the bb files needed for bbappend files or it may lead to other unwanted behavior.

I haven't been able to find a good solution for this. It seems that the various layer.confs are parsed globally once, at the start, and not per multiconfig.

Any ideas are welcome.