This is the result of bug 272676, but the warning is senseless for vanilla-sources. It's probably also senseless for gentoo-sources, as I don't see the patch there anymore. The code can be found in eclass/kernel-2.eclass.
commit d2cfdf5fd9db37677421c900f330d58a8674de13 (HEAD -> master, origin/master, origin/HEAD) Author: Mike Pagano <mpagano@gentoo.org> Date: Mon Jun 18 14:54:29 2018 -0400 kernel-2.eclass: Fix the unapplying of fbcondecor for sparc and include check for K_WANT_GENPATCHES. Closes bug #652152.