bootloader: fix missing typedef on enum defintion

(MINOR CHANGES)
Closes https://github.com/espressif/esp-idf/pull/2140

components/bootloader_support/include/esp_image_format.h

@@ -36,7 +36,7 @@ typedef enum {
 } esp_image_spi_mode_t;
 
 /* SPI flash clock frequency */
-enum {
+typedef enum {
     ESP_IMAGE_SPI_SPEED_40M,
     ESP_IMAGE_SPI_SPEED_26M,
     ESP_IMAGE_SPI_SPEED_20M,