binman: Add an entry for a Chromium vblock

This adds support for a Chromium verified boot block, used to sign a
read-write section of the image.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/entry.py b/tools/binman/entry.py
index 8b910fe..996f03e 100644
--- a/tools/binman/entry.py
+++ b/tools/binman/entry.py
@@ -65,7 +65,7 @@
         self.name = node and (name_prefix + node.name) or 'none'
         self.offset = None
         self.size = None
-        self.data = ''
+        self.data = None
         self.contents_size = 0
         self.align = None
         self.align_size = None