splash_source: add support for filesystem formatted usb

Add support for loading splash image from USB drive formatted with a
filesystem.

Cc: Igor Grinberg <grinberg@compulab.co.il>
Cc: Tom Rini <trini@konsulko.com>
Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
diff --git a/include/splash.h b/include/splash.h
index d1fba69..b728bd6 100644
--- a/include/splash.h
+++ b/include/splash.h
@@ -28,6 +28,7 @@
 	SPLASH_STORAGE_NAND,
 	SPLASH_STORAGE_SF,
 	SPLASH_STORAGE_MMC,
+	SPLASH_STORAGE_USB,
 };
 
 enum splash_flags {