I am not exactly sure but my searching took me to
/usr/src/share/mk/bsd.kmod.mk
From there I did a search on
kmod.mk and discovered relevant files in
/usr/src/sys/conf/kmod.mk
I am short on time but I would gander that there is something relevant in this directory.
In this post some users show how to add modules to make.conf:
Hello :) It would be nice that experienced users shared their make.conf and src.conf to help new users like me :) I know that there are man pages and I read both of them, but real user's configurations are IMHO also helpful!
forums.freebsd.org
# Modules to build instead of all
# MODULES_OVERRIDE = linux
# Modules not to build
# WITHOUT_MODULES = linux acpi
Usually anytime there is a WITHOUT_ option there is also a corresponding WITH_ option.