When TARGET_ARCH_${kernel} gets set, it is using the host's config,

not the one we build as part of make world.  This means that make
universe will fail if building on a too-old current or any stable
system prior to a few days ago in weird ways (parse errors from
shell).  This copes with these old systems in two ways:

(1) Works around the WARNING: issue by filtering all warnings that
    sneak onto stdout.

(2) if TARGET_ARCH_${kernel} winds up being empty, then we error out
    immediately with a semi-useful error message.  This usually comes from
    config not groking -m.

Ideally, we'd use a buildworld's config here, but that's tricky, so
I'll leave that detail to others to fix (it has to be done post make
world for the arch rather than at the top level makefile).  This
should make 'make universe' usable from recent 8-stable systems
(recent == last few months or so) for building -current.  They have
-m, but spewed warnings out stdout.  Older systems will now at least
get a firm error early rather than a confusing error late.

Makefile

@@ -351,7 +351,10 @@ universe_kernconfs:
 .for kernel in ${KERNCONFS}
 TARGET_ARCH_${kernel}!=	cd ${.CURDIR}/sys/${TARGET}/conf && \
 	config -m ${.CURDIR}/sys/${TARGET}/conf/${kernel} 2> /dev/null | \
-	cut -f 2
+	grep -v WARNING: | cut -f 2
+.if empty(TARGET_ARCH_${kernel})
+.error "Target architecture for ${TARGET}/conf/${kernel} unknown.  config(8) likely too old."
+.endif
 universe_kernconfs: universe_kernconf_${TARGET}_${kernel}
 universe_kernconf_${TARGET}_${kernel}:
 	@(cd ${.CURDIR} && env __MAKE_CONF=/dev/null \