stmhal: Put flash in deep power-down mode when entering stop mode.

This can get PYBv1.0 stop current down to around 290uA.

stmhal/Makefile

@@ -168,6 +168,7 @@ SRC_HAL = $(addprefix $(HAL_DIR)/src/,\
 	stm32f4xx_hal_pcd.c \
 	stm32f4xx_hal_pcd_ex.c \
 	stm32f4xx_hal_pwr.c \
+	stm32f4xx_hal_pwr_ex.c \
 	stm32f4xx_hal_rcc.c \
 	stm32f4xx_hal_rcc_ex.c \
 	stm32f4xx_hal_rng.c \

stmhal/modpyb.c

@@ -425,6 +425,9 @@ STATIC MP_DEFINE_CONST_FUN_OBJ_1(pyb_udelay_obj, pyb_udelay);
 
 /// \function stop()
 STATIC mp_obj_t pyb_stop(void) {
+    // takes longer to wake but reduces stop current
+    HAL_PWREx_EnableFlashPowerDown();
+
     HAL_PWR_EnterSTOPMode(PWR_LOWPOWERREGULATOR_ON, PWR_STOPENTRY_WFI);
 
     // reconfigure the system clock after waking up