spl: Add a debug string before the jump to U-Boot

As a debug option, add positive confirmation that SPL has completed
execution. This can help with diagnosing the location of unexpected hangs.

Signed-off-by: Simon Glass <sjg@chromium.org>

common/spl/spl.c

@@ -298,6 +298,7 @@ void board_init_r(gd_t *dummy1, ulong dummy2)
 	      gd->malloc_ptr / 1024);
 #endif
 
+	debug("loaded - jumping to U-Boot...");
 	jump_to_image_no_args(&spl_image);
 }