Add a new makefile rule to check for build errors.

This rule will build each program, ignore all output, and return pass or
fail depending on whether the build has errors. This is helpful for
finding out which programs need to be fixed when a new Rust nightly
build inevitably breaks everything.

Makefile

@@ -374,4 +374,15 @@ busytest: $(BUILDDIR)/busybox $(BUILDDIR)/.config
 	(cd $(BUSYBOX_SRC)/testsuite && bindir=$(BUILDDIR) ./runtest $(RUNTEST_ARGS))
 endif
 
+# This rule will build each program, ignore all output, and return pass
+# or fail depending on whether the build has errors.
+build-check:
+	@for prog in $(sort $(PROGS)); do \
+		make BUILD="$$prog" >/dev/null 2>&1; status=$$?; \
+		if [ $$status -eq 0 ]; \
+		then printf "%-10s\t\033[1;32mpass\033[00;m\n" $$prog; \
+		else printf "%-10s\t\033[1;31mfail\033[00;m\n" $$prog; \
+		fi; \
+	done
+
 .PHONY: $(TEMPDIR) all deps test distclean clean busytest install uninstall