Thank you for replying, but I still can't see the connection.
Just for example, the ".bin" file extension on the end of openwrt-lantiq-xrx200-bt_homehub-v5a-squashfs-sysupgrade.bin. I can't find where that file extension is defined. It certainly doesn't appear to be in image.mk#L36-L42 or in Config-devel.in#L48-L54 or in image-config.in#L146-L267 (the latter 2 references are from your "See also").
(Please understand that I'm only giving the file extension, bin, as an example of a literal string that I can't find - I don't want to change it, I just want to find it and learn more about the build process as I go.)
I'm starting to think that the output filename may be defined somewhere under target/linux/lantiq/image but I'm still not sure.