Add name field to external configuration

The fetch_externals.py script does not detect if the 'fetchurl' field
changes in the 'externals.json' file. To handle this an external
repo can be assigeded a 'name' field, which allows the remote to be
updated.

Change-Id: I175f7811929baafad84e0c46c7cdd273b91549b1
diff --git a/externals.json b/externals.json
index 67adc9a..ad9e62c 100644
--- a/externals.json
+++ b/externals.json
@@ -41,8 +41,9 @@
         },
         {
             "path": "core_software/rtos/threadx",
-            "fetchurl": "https://github.com/azure-rtos/threadx",
-            "revision": "master"
+            "name": "linaro",
+            "fetchurl": "https://github.com/Linaro/threadx",
+            "revision": "mps3_an547"
         },
         {
             "path": "core_software/tflite_micro",