spi_flash: add config option to override flash size in bootloader header

Sometimes the flash size read from bootloader is not correct. This may
forbid SPI Flash driver from reading the the area larger than the size
in bootloader header.

When the new config option is enabled, the latest configured
ESPTOOLPY_FLAHSIZE in the app header will be used to override the value
read from bootloader header.

components/bootloader_support/include/esp_image_format.h

@@ -169,6 +169,14 @@ esp_err_t esp_image_verify_bootloader(uint32_t *length);
  */
 esp_err_t esp_image_verify_bootloader_data(esp_image_metadata_t *data);
 
+/**
+ * @brief Get the flash size of the image
+ *
+ * @param app_flash_size The value configured in the image header
+ * @return Actual size, in bytes.
+ */
+int esp_image_get_flash_size(esp_image_flash_size_t app_flash_size);
+
 
 typedef struct {
     uint32_t drom_addr;