Switching FreeRTOS-Kernel branch

Switch FreeRTOS-Kernel from master to main branch.

Change-Id: I298f7423b23de76e0d40a7faf0c1d9efdd8502e6
diff --git a/externals.json b/externals.json
index b79c418..d06dc84 100644
--- a/externals.json
+++ b/externals.json
@@ -32,7 +32,7 @@
         {
             "path": "core_software/rtos/freertos/freertos_kernel",
             "fetchurl": "https://github.com/FreeRTOS/FreeRTOS-Kernel.git",
-            "revision": "master"
+            "revision": "main"
         },
         {
             "path": "core_software/tensorflow",