npx is no more mandatory than sc. Its mandatoryness went away in
rev.1.226 of i386/machdep.c 9 months before it was made mandatory in
rev.1.24 of config/mkmakefile.c.
This change is mainly to test building of minimal kernel configurations.
npx should really be even more standard than clk. It was optional mainly
so that the usual device driver configuration info could be specified in
the usual way in config files, but this hasn't been necessary for a few
years.
c920faa62e