Makefile: Fix make clean

The line respective to the mgmt was building it instead of cleaning
it up.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>

Makefile

@@ -34,7 +34,7 @@ $(submake) $(modulemk): Makefile
 
 clean:
 	$(call quiet, rm -rf build/$(mode), CLEAN)
-	$(call only-if, $(mgmt), $(MAKE) -C mgmt)
+	$(call only-if, $(mgmt), $(call quiet, $(MAKE) -C mgmt clean >> /dev/null, MGMT CLEAN))
 	$(call quiet, cd java && mvn clean -q, MVN CLEAN)
 
 check: export image ?= tests