spl: common: Support for USB MSD FAT image loading

Add SPL support to be able to detect a USB Mass Storage device
connected to a USB host.  Once a USB Mass storage device is detected
the SPL will load the u-boot.img from a FAT partition to target address.

Signed-off-by: Dan Murphy <dmurphy@ti.com>

include/spl.h

@@ -61,6 +61,9 @@ void spl_spi_load_image(void);
 /* Ethernet SPL functions */
 void spl_net_load_image(const char *device);
 
+/* USB SPL functions */
+void spl_usb_load_image(void);
+
 /* SPL FAT image functions */
 int spl_load_image_fat(block_dev_desc_t *block_dev, int partition, const char *filename);
 int spl_load_image_fat_os(block_dev_desc_t *block_dev, int partition);