From: Justin Forbes on gitlab.com
Right, so the Makefile generates vmlinux.h if the value is null, and
copies it over if the value is set to something. We could probably
simplify this even more. I assumed there was a flag passed in cki to
rpmbuild for --only-tools or something, but I don't see it. If we had
such a flag, we could key off of that and set VMLINUX_H to the path for
this specific kernel in kernel-devel. If we are building without that
flag, it would be null and then generate it's own.