Add flatbuffers

Change-Id: I2eed448ef31b54a1e8258dcbc9e384fd85201c05
diff --git a/externals.json b/externals.json
index 6ee5456..3dbba48 100644
--- a/externals.json
+++ b/externals.json
@@ -30,6 +30,11 @@
             "revision": "master"
         },
         {
+            "path": "linux_driver_stack/flatbuffers",
+            "fetchurl": "https://github.com/google/flatbuffers",
+            "revision": "master"
+        },
+        {
             "path": "vela",
             "fetchurl": "https://review.mlplatform.org/ml/ethos-u/ethos-u-vela",
             "pushurl": "ssh://review.mlplatform.org:29418/ml/ethos-u/ethos-u-vela",