scripts: add selftest layout to disk_layout_anthos.json

The 'selftest' layout is required when building kselftest images.
Add it to disk_layout_anthos.json so that boards inheriting from it
(anthos, arion, pholus) can successfully build kselftest images.

BUG=None
TEST=disk_layout_tool validate selftest src/private-overlays/overlay-arion-private/scripts/disk_layout.json
RELEASE_NOTE=None

Change-Id: I1a216ea3ffabb13f79dfabd3d11fcd7199131dc3
Reviewed-on: https://cos-review.googlesource.com/c/third_party/platform/crosutils/+/174167
Tested-by: Chenglong Tang <chenglongtang@google.com>
Reviewed-by: Angel Adetula <angeladetula@google.com>

build_library/disk_layout_anthos.json

diff --git a/build_library/disk_layout_anthos.json b/build_library/disk_layout_anthos.json
index db4cf1c..ed62182 100644
--- a/build_library/disk_layout_anthos.json
+++ b/build_library/disk_layout_anthos.json
@@ -250,6 +250,18 @@
         "size": "16384 MiB",
         "fs_size": "16000 MiB"
       }
+    ],
+    "selftest": [
+      {
+        # KERN-A partition
+        "num": 2,
+        "size": "32 MiB"
+      },
+      {
+        # KERN-B partition
+        "num": 4,
+        "size": "32 MiB"
+      }
     ]
   }
 }