stm32/boards/PYBD_SFx: Configure EN_3V3 pin as output on boot.

But leave it turned off, the application must turn it on if/when needed.

ports/stm32/boards/PYBD_SF2/board_init.c

@@ -33,6 +33,9 @@ void mboot_board_early_init(void) {
 }
 
 void board_early_init(void) {
+    // Configure EN_3V3 as an output pin, but keep it turned off
+    mp_hal_pin_config(pyb_pin_EN_3V3, MP_HAL_PIN_MODE_OUTPUT, MP_HAL_PIN_PULL_NONE, 0);
+
     // Explicitly init SPI2 because it's not enabled as a block device
     spi_bdev_ioctl(&spi_bdev2, BDEV_IOCTL_INIT, (uint32_t)&spiflash2_config);
 }