[build] Provide mechanism for listing constituent object sizes

You can now type e.g.

  make bin/rtl8139.rom.sizes

in order to see the (uncompressed) sizes of all of the object files
linked in to bin/rtl8139.rom.  This should make it easier to identify
relevant code bloat.

src/Makefile.housekeeping

@@ -657,6 +657,8 @@ define objs_list
 endef
 $(BIN)/%.objs : $(BIN)/%.tmp
 	$(Q)$(ECHO) $(call objs_list,$<)
+$(BIN)/%.sizes : $(BIN)/%.tmp
+	$(Q)$(SIZE) -t $(foreach OBJ,$(call objs_list,$<),$(BIN)/$(OBJ).o)
 
 # Get dependency list for the specified target
 #