Add FreeRTOS repositories

Change-Id: I63bf3b2c587ace9437248a6be2bc7aeb2878f9cb
diff --git a/externals.json b/externals.json
index e9dfc4f..b79c418 100644
--- a/externals.json
+++ b/externals.json
@@ -25,6 +25,16 @@
             "revision": "develop"
         },
         {
+            "path": "core_software/rtos/freertos",
+            "fetchurl": "https://github.com/aws/amazon-freertos.git",
+            "revision": "master"
+        },
+        {
+            "path": "core_software/rtos/freertos/freertos_kernel",
+            "fetchurl": "https://github.com/FreeRTOS/FreeRTOS-Kernel.git",
+            "revision": "master"
+        },
+        {
             "path": "core_software/tensorflow",
             "fetchurl": "https://github.com/tensorflow/tensorflow",
             "revision": "master"