stm32/Makefile: Automatically rebuild if make USE_MBOOT=0/1 changed.

ports/stm32/Makefile

@@ -714,6 +714,9 @@ SRC_QSTR_AUTO_DEPS += $(GEN_CDCINF_HEADER)
 # which source files might need it.
 $(OBJ): | $(GEN_PINS_HDR)
 
+# Automatically rebuild if specific settings are changed on command line
+$(OBJ): $(call depend_var, USE_MBOOT)
+
 # With conditional pins, we may need to regenerate qstrdefs.h when config
 # options change.
 $(HEADER_BUILD)/qstrdefs.generated.h: $(BOARD_DIR)/mpconfigboard.h