sys/Makefile: build all sys modules with nonrecursive makefiles

sys/Makefile

@@ -1,7 +1,14 @@
+OLD_NONRECURSIVE := $(NONRECURSIVE)
+NONRECURSIVE = 1
+# First include for current module (should be done before any 'include')
+include $(RIOTBASE)/Makefile.base
+
+# Then take care of other modules
+
 include Makefile.modules
 include $(wildcard */Makefile.modules)
 include $(wildcard */*/Makefile.modules)
 
-DIRS += $(foreach module, $(USEMODULE), $(MODULE_DIR_$(module)))
-
-include $(RIOTBASE)/Makefile.base
+MAKEFILE_DIRS = $(sort $(foreach module, $(USEMODULE), $(MODULE_DIR_$(module))))
+include $(MAKEFILE_DIRS:%=%/Makefile)
+NONRECURSIVE := $(OLD_NONRECURSIVE)