conversion of some constants to variables, this is identical to the code
that was previously present in both distrib/amd64/kmod/Makefile and
distrib/i386/kmod/Makefile.
Change distrib/amd64/kmod/Makefile and distrib/i386/kmod/Makefile to just
set some variables and .include "../../common/Makefile.minirootkmod".