boot configuration files from NOR Flash

Why do you need both EPCS and a NOR flash for configuration?

I can't find that flash part name. I guess it's a parallel flash; then you could possibly connect the switch signal to one of the higher address lines and make a true 'hardware' selection of the image.

On the other hand the usual solution is using remote update core: you always load a third default configuration, which only purpose is testing the switch status and triggering a fpga reconfiguration with the appropriate image.