Add a hack to disable AUTO_OBJ for 'old style' kernel builds.

Another solution would be to extend the Makefile.sys.inc idea, or a .no_obj
file, to more places but I would rather keep that limited to the top-level
build for now to not impact performance (statting a file in every make call)
or to bring unintended side-effects.

Reported by: jhb, imp
Sponsored by: Dell EMC Isilon