binman: Update missing optional external blob warning text

Make it more clear that the missing external blob is optional in the
printed warning message.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/ftest.py b/tools/binman/ftest.py
index 1cfa349..36c72be 100644
--- a/tools/binman/ftest.py
+++ b/tools/binman/ftest.py
@@ -6390,7 +6390,7 @@
         err = stderr.getvalue()
         self.assertRegex(
             err,
-            "Image '.*' is missing external blobs but is still functional: missing")
+            "Image '.*' is missing optional external blobs but is still functional: missing")
 
     def testSectionInner(self):
         """Test an inner section with a size"""