Add signature support

Change-Id: Idf5b6144b78e156e6c77e656cfe14097750259e5
diff --git a/dtbhtool/bootimg.h b/dtbhtool/bootimg.h
index 4692f0c..ef0e61c 100644
--- a/dtbhtool/bootimg.h
+++ b/dtbhtool/bootimg.h
@@ -61,6 +61,8 @@
 ** +-----------------+
 ** | device tree     | p pages
 ** +-----------------+
+** | signature       | 256 bytes
+** +-----------------+
 **
 ** n = (kernel_size + page_size - 1) / page_size
 ** m = (ramdisk_size + page_size - 1) / page_size