test: spl: Add a test for the MMC load method

Add a test for the MMC load method. This shows the general shape of tests
to come: The main test function calls do_spl_test_load with an appropriate
callback to write the image to the medium.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/include/spl.h b/include/spl.h
index 03a4a83..ad1543a 100644
--- a/include/spl.h
+++ b/include/spl.h
@@ -673,6 +673,10 @@
 	}
 #endif
 
+#define SPL_LOAD_IMAGE_GET(_priority, _boot_device, _method) \
+	ll_entry_get(struct spl_image_loader, \
+		     _boot_device ## _priority ## _method, spl_image_loader)
+
 /* SPL FAT image functions */
 
 /**