corundum: add lightclean target

preserves the object file for the verilator design, as this takes
forever to compile.

corundum/Makefile

@@ -39,7 +39,9 @@ corundum_verilator: obj_dir/Vinterface
 #	    obj_dir/Vinterface__Trace__Slow.o
 #	$(CXX) $(CXXFLAGS) $(LDFLAGS) -o $@ $^ $(LDLIBS)
 
+lightclean:
+	rm -f obj_dir/Vinterface corundum_verilator obj_dir/mem.o obj_dir/dma.o
 clean:
 	rm -rf obj_dir corundum_verilator *.o
 
-.PHONY: all clean
+.PHONY: all clean lightclean