outside */dist/* subdirectories.
When USE_PIGZGZIP=yes, bsd.own.mk sets TOOL_GZIP=${TOOL_PIGZ},
so there's no need to test USE_PIGZGZIP in these Makefiles.
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".