stm32/mboot: Update dependencies to enable parallel build with -j.

ports/stm32/mboot/Makefile

@@ -178,7 +178,7 @@ GEN_PINS_AF_CONST = $(HEADER_BUILD)/pins_af_const.h
 GEN_PINS_AF_DEFS = $(HEADER_BUILD)/pins_af_defs.h
 GEN_PINS_AF_PY = $(BUILD)/pins_af.py
 
-$(BUILD)/main.o: $(GEN_QSTRDEFS_GENERATED) $(GEN_PINS_AF_DEFS)
+$(OBJ): $(GEN_QSTRDEFS_GENERATED) $(GEN_PINS_AF_DEFS)
 
 $(HEADER_BUILD):
 	$(MKDIR) -p $(BUILD)/genhdr