Add initial update-dependencies script.

This script can be used to automatically update dotnet/cli's
dependencies on other repos like CoreFX, NuGet, etc.

Currently the script supports updating the CoreFX dependencies.

scripts/update-dependencies/Dirs.cs

@@ -0,0 +1,12 @@
+// Copyright (c) .NET Foundation and contributors. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using System.IO;
+
+namespace Microsoft.DotNet.Scripts
+{
+    public static class Dirs
+    {
+        public static readonly string RepoRoot = Directory.GetCurrentDirectory();
+    }
+}