binman: Support lists of external blobs

Sometimes it is useful to have a list of related external blobs in a
single entry. An example is the DDR binaries used by meson. There are
9 files in total. Add support for this, so we don't have to have a
separate entry for each.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/entries.rst b/tools/binman/entries.rst
index 2ebac51..d5aa3b0 100644
--- a/tools/binman/entries.rst
+++ b/tools/binman/entries.rst
@@ -69,6 +69,20 @@
 
 
 
+Entry: blob-ext-list: List of externally built binary blobs
+-----------------------------------------------------------
+
+This is like blob-ext except that a number of blobs can be provided,
+typically with some sort of relationship, e.g. all are DDC parameters.
+
+If any of the external files needed by this llist is missing, binman can
+optionally ignore it and produce a broken image with a warning.
+
+Args:
+    filenames: List of filenames to read and include
+
+
+
 Entry: blob-named-by-arg: A blob entry which gets its filename property from its subclass
 -----------------------------------------------------------------------------------------