Hello Simon,
Well some build systems may take the u-boot.dtb and package it up with U-Boot in a different way. Also they may look inside this file to find out things about the board they are targeting. For example, you might imagine a flash program which gets flash programming parameters from this file so it knows how to flash the board.
I don't consider something in the dts/ directory to be a build product really. I suspect one day we might support multiple .dtb files in U-Boot and select between them based on a setting or a hardware feature, and in that case perhaps everything will be put in a FIT, but we are not there yet.
But for now I see the .dtb file as a first-class build product just like the main U-Boot binary.
Then, I agreed to include this version to the main line. Thanks for your answering.
I confirmed this patch can apply on commit 1674df60 + Follow-up for Kbuild series v2.
Best Regards Masahiro Yamada