binman: Allow listing an image created by a newer version

If an older version of binman is used to list images created by a newer
one, it is possible that it will contain entry types that are not
supported. At present this produces an error.

Adjust binman to use a plain 'blob' entry type to cope with this, so the
image can at least be listed.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/binman.rst b/tools/binman/binman.rst
index 210d0c5..26f462a 100644
--- a/tools/binman/binman.rst
+++ b/tools/binman/binman.rst
@@ -913,6 +913,11 @@
           u-boot-dtb        180   108  u-boot-dtb        80          3b5
       image-header          bf8     8  image-header     bf8
 
+If an older version of binman is used to list images created by a newer one, it
+is possible that it will contain entry types that are not supported. These still
+show with the correct type, but binman just sees them as blobs (plain binary
+data). Any special features of that etype are not supported by the old binman.
+
 
 Extracting files from images
 ----------------------------