meta-phosphor: generate supported machine list

Add a script to generate a list of supported machines and hook it
into the `run-repotest` to ensure that it is ran as necessary when
new machines are added or removed.

Fixes openbmc/openbmc#3926.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: If33aa08451bb8a789459bf49d8d7ec9044973304
diff --git a/meta-phosphor/scripts/run-repotest b/meta-phosphor/scripts/run-repotest
index dee4439..8458236 100755
--- a/meta-phosphor/scripts/run-repotest
+++ b/meta-phosphor/scripts/run-repotest
@@ -164,4 +164,10 @@
 
 done
 
+# Regenerate supported machines list.
+meta-phosphor/scripts/generate-machine-list
+if ! git --no-pager diff --exit-code ; then
+    echo "Changes to supported-machines.md necessary due to new meta-layer support."
+fi
+
 echo "Repo test passed"