fdt: Provide debug info when a device tree cannot be found

It can be quite confusing with a new platform to figure out why the device
tree cannot be located. Add some debug information for this case.

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

lib/fdtdec.c

@@ -568,6 +568,13 @@ int fdtdec_prepare_fdt(void)
 		puts("Missing DTB\n");
 #else
 		puts("No valid device tree binary found - please append one to U-Boot binary, use u-boot-dtb.bin or define CONFIG_OF_EMBED. For sandbox, use -d <file.dtb>\n");
+# ifdef DEBUG
+		if (gd->fdt_blob) {
+			printf("fdt_blob=%p\n", gd->fdt_blob);
+			print_buffer((ulong)gd->fdt_blob, gd->fdt_blob, 4,
+				     32, 0);
+		}
+# endif
 #endif
 		return -1;
 	}