binman: Tidy up the vblock entry

At present if there are two vblock entries an image their contents are
written to the same file in the output directory. This prevents checking
the contents of each separately.

Fix this by adding part of the entry path to the filename, and add some
missing comments.

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

tools/binman/ftest.py

@@ -1316,7 +1316,7 @@ class TestFunctional(unittest.TestCase):
         """Fake calls to the futility utility"""
         if pipe_list[0][0] == 'futility':
             fname = pipe_list[0][3]
-            with open(fname, 'w') as fd:
+            with open(fname, 'wb') as fd:
                 fd.write(VBLOCK_DATA)
             return command.CommandResult()