FWIW, I fully support going this direction. I've been making some steps in the same direction, but as always there are more things to do than time to do them in.
My advice: start small and don't try to change too much at once. Get the basic infrastructure in place and get the patches out to the list. Maybe start with being able to select the board port with Kconfig while leaving include/configs/* in place. Once that is working and proven, then start migrating configuation over to Kconfig.
I think it might be worthwhile to see if we long term can merge the configuration with the Linux configuration
I think some of the configuration items could be the same and could therefore use the same name.
Will U-Boot break, if the Linux .config is included in the Makefile?
Best Regards Ulf Samuelsson