Makefile.include: Add --cref to linker options

The --cref option generates a detailed cross reference list in the map file.
Useful for checking what functions are referenced from where.

Makefile.include

@@ -188,7 +188,7 @@ ifeq (,$(RIOTNOLINK))
 ifeq ($(BUILDOSXNATIVE),1)
 	$(AD)$(if $(CPPMIX),$(CXX),$(LINK)) $(UNDEF) -o $(ELFFILE) $(BASELIBS) $(LINKFLAGS) $(LINKFLAGPREFIX)-no_pie
 else
-	$(AD)$(if $(CPPMIX),$(CXX),$(LINK)) $(UNDEF) -o $(ELFFILE) $(LINKFLAGPREFIX)--start-group $(BASELIBS) -lm $(LINKFLAGPREFIX)--end-group  $(LINKFLAGPREFIX)-Map=$(BINDIR)$(APPLICATION).map $(LINKFLAGS)
+	$(AD)$(if $(CPPMIX),$(CXX),$(LINK)) $(UNDEF) -o $(ELFFILE) $(LINKFLAGPREFIX)--start-group $(BASELIBS) -lm $(LINKFLAGPREFIX)--end-group  $(LINKFLAGPREFIX)-Map=$(BINDIR)$(APPLICATION).map $(LINKFLAGPREFIX)--cref $(LINKFLAGS)
 endif
 	$(AD)$(SIZE) $(ELFFILE)
 	$(AD)$(OBJCOPY) $(OFLAGS) $(ELFFILE) $(HEXFILE)