loader/u-boot: fix alignment of uimage components.

The "blobs" in a U-Boot uimage are aligned at 4 bytes, which we
did not take into account. Found this when adding a 3rd blob
containing the Flattened Device Tree for ARM.

src/system/boot/platform/u-boot/uimage.cpp

@@ -55,7 +55,7 @@ image_multi_getimg(struct image_header *image, uint32 idx, uint32 *data, uint32
 			*size = ntohl(sizes[i]);
 			return true;
 		}
-		base += ntohl(sizes[i]);
+		base += (ntohl(sizes[i]) + 3) & ~3;
 	}
 	return false;
 }