See patch.
The actual patch is buildrom_update_kconfig.patch.gz but that's huge and totally unreadable so I recommend reviewing buildrom_update_kconfig_makefile.patch
That's the only change to the buildrom code itself, the rest is just replacing the whole scripts/kconfig/ dir with the new kconfig.
I used the coreinfo kconfig/ with only a handful of string replacements a la s/coreinfo/buildrom/ so the diff to coreinfo's kconfig is tiny.
I did some ad-hoc testing with this, tried various kconfig targets (menuconfig, xconfig, gconfig, ...) and various buildrom builds and it seems to work fine so far, but some more testing by others is highly appreciated.
Thanks, Uwe.