Makefile: fix distclean and *_config rules to be less noise

Signed-off-by: Lauri Hintsala <lauri.hintsala@bluegiga.com>

Makefile

@@ -643,7 +643,7 @@ unconfig:
 
 sinclude $(obj).boards.depend
 $(obj).boards.depend:	boards.cfg
-	awk '(NF && $$1 !~ /^#/) { print $$1 ": " $$1 "_config; $$(MAKE)" }' $< > $@
+	@awk '(NF && $$1 !~ /^#/) { print $$1 ": " $$1 "_config; $$(MAKE)" }' $< > $@
 
 #
 # Functions to generate common board directory names