bootm: Use the error return from boot_get_kernel()

Rather than looking for a zero-sized image, use the error code returned
to determine if things are OK.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/boot/bootm.c b/boot/bootm.c
index 6ed60bf..a23c791 100644
--- a/boot/bootm.c
+++ b/boot/bootm.c
@@ -324,9 +324,9 @@
 	/* get kernel image header, start address and length */
 	ret = boot_get_kernel(cmdtp->name, argv[0], &images,
 			      &images.os.image_start, &images.os.image_len,
-		       &os_hdr);
-	if (images.os.image_len == 0) {
-		puts("ERROR: can't get kernel image!\n");
+			      &os_hdr);
+	if (ret) {
+		printf("ERROR %dE: can't get kernel image!\n", ret);
 		return 1;
 	}