ci/listmachines: add helper to list machines in a layer

Instead of duplicating this logic, put it in a little helper module.

Change-Id: I2ca881c8de7e6ec6d4a940255d9ba97226e78d87
Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>

ci/listmachines.py

@@ -0,0 +1,29 @@
+#! /usr/bin/env python3
+
+import pathlib
+import typing
+import sys
+
+"""
+List all of the machines available under the listed sub-layers of meta-arm.
+"""
+def list_machines(layers: typing.Sequence[str]) -> typing.Set[str]:
+    machines = set()
+
+    # We know we're in meta-arm/scripts, so find the top-level directory
+    metaarm = pathlib.Path(__file__).resolve().parent.parent
+    if metaarm.name != "meta-arm":
+        raise Exception("Not running inside meta-arm")
+    
+    for layer in layers:
+        machines |= set(p.stem for p in (metaarm / layer / "conf" / "machine").glob("*.conf"))
+    return machines
+
+if __name__ == "__main__":
+    if len(sys.argv) > 1:
+        machines = list_machines(sys.argv[1:])
+        print(" ".join(sorted(machines)))
+        sys.exit(0)
+    else:
+        print("Usage:\n$ %s [layer name ...] " % sys.argv[0])
+        sys.exit(1)