Update branch names in external configuration

Some repositories renamed master branch to main, update the
revisions to reflect this. No functional change.

Signed-off-by: Jonny Svärd <jonny.svaerd@arm.com>
Change-Id: I6ffd0ab9e323e950f6c1c82b851f48fb806d1bef
diff --git a/externals.json b/externals.json
index 2b4aec8..10a66eb 100644
--- a/externals.json
+++ b/externals.json
@@ -5,19 +5,19 @@
             "path": "core_platform",
             "fetchurl": "https://review.mlplatform.org/ml/ethos-u/ethos-u-core-platform",
             "pushurl": "ssh://review.mlplatform.org:29418/ml/ethos-u/ethos-u-core-platform",
-            "revision": "master"
+            "revision": "main"
         },
         {
             "path": "core_software",
             "fetchurl": "https://review.mlplatform.org/ml/ethos-u/ethos-u-core-software",
             "pushurl": "ssh://review.mlplatform.org:29418/ml/ethos-u/ethos-u-core-software",
-            "revision": "master"
+            "revision": "main"
         },
         {
             "path": "core_software/core_driver",
             "fetchurl": "https://review.mlplatform.org/ml/ethos-u/ethos-u-core-driver",
             "pushurl": "ssh://review.mlplatform.org:29418/ml/ethos-u/ethos-u-core-driver",
-            "revision": "master"
+            "revision": "main"
         },
         {
             "path": "core_software/cmsis",
@@ -63,7 +63,7 @@
             "path": "linux_driver_stack",
             "fetchurl": "https://review.mlplatform.org/ml/ethos-u/ethos-u-linux-driver-stack",
             "pushurl": "ssh://review.mlplatform.org:29418/ml/ethos-u/ethos-u-linux-driver-stack",
-            "revision": "master"
+            "revision": "main"
         },
         {
             "path": "vela",