cmd/tailscaled, ipn/conffile: support ec2 user-data config file

Updates #1412
Updates #1866

Change-Id: I4d08fb233b80c2078b3b28ffc18559baabb4a081
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

omit/aws_def.go

@@ -0,0 +1,9 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !ts_omit_aws
+
+package omit
+
+// AWS is whether AWS support should be omitted from the build.
+const AWS = false

omit/aws_omit.go

@@ -0,0 +1,9 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build ts_omit_aws
+
+package omit
+
+// AWS is whether AWS support should be omitted from the build.
+const AWS = true

omit/omit.go

@@ -0,0 +1,12 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package omit provides consts to access Tailscale ts_omit_FOO build tags.
+// They're often more convenient to eliminate some away locally with a const
+// rather than using build tags.
+package omit
+
+import "errors"
+
+// Err is an error that can be returned by functions in this package.
+var Err = errors.New("feature not linked into binary per ts_omit build tag")