Yes, I did copy-over site-local/Kconfig, but this initially had no effect on FSP_HEADER_PATH in the resulting .config. Hmm - apparently, the site-local/Kconfig does not override an existing .config setting, except for variables that have not already been set. So some settings change after copying-over site-local/Kconfig, and some do not change. It's not obvious what to expect at first, though it seems obvious in retrospect.
Right, Kconfig never changes a setting in an existing .config based on a different default value. To get it to update values to new default settings, run:
make savedefconfig && make defconfig KBUILD_DEFCONFIG=${PWD}/defconfig
The Kconfig in coreboot document is written with developers writing Kconfig files in mind, but it's got a lot of useful information about how Kconfig works.
https://doc.coreboot.org/getting_started/kconfig.html
Martin