makefiles: fix duplicate for missing features

makefiles/info-global.inc.mk

@@ -26,7 +26,7 @@ define board_missing_features
 
   include $(RIOTBASE)/Makefile.dep
 
-  FEATURES_MISSING := $$(filter-out $$(FEATURES_PROVIDED), $$(FEATURES_REQUIRED))
+  FEATURES_MISSING := $$(sort $$(filter-out $$(FEATURES_PROVIDED), $$(FEATURES_REQUIRED)))
   ifneq (, $${FEATURES_MISSING})
     BOARDS_FEATURES_MISSING += "${1} $${FEATURES_MISSING}"
     ifneq (, $$(filter-out $$(FEATURES_OPTIONAL), $$(FEATURES_MISSING)))

makefiles/info.inc.mk

@@ -115,4 +115,4 @@ info-cpu:
 	@echo $(CPU)
 
 info-features-missing:
-	@echo $(filter-out $(FEATURES_PROVIDED), $(FEATURES_REQUIRED))
+	@for i in $(sort $(filter-out $(FEATURES_PROVIDED), $(FEATURES_REQUIRED))); do echo $$i; done