make: factor out features handling, add default feature handling

Makefile.features

+# import list of provided features
+-include $(RIOTBOARD)/$(BOARD)/Makefile.features
+-include $(RIOTCPU)/$(CPU)/Makefile.features
+
+DEFAULT_FEATURES += periph_pm
+
+# add available default features to required list
+FEATURES_REQUIRED += $(filter-out $(DISABLE_FEATURES), $(filter $(FEATURES_PROVIDED), $(DEFAULT_FEATURES)))

Makefile.include

@@ -398,8 +398,8 @@ $(CURDIR)/eclipsesym.xml:
 # Extra make goals for testing and comparing changes.
 include $(RIOTBASE)/Makefile.buildtests
 
-# import list of provided features
--include $(RIOTBOARD)/$(BOARD)/Makefile.features
+# process provided features
+include $(RIOTBASE)/Makefile.features
 
 # Export variables used throughout the whole make system:
 include $(RIOTBASE)/Makefile.vars