Introduce has_sev attribute

Signed-off-by: Roman Volosatovs <[email protected]>

Author: rvolosatovs

build.rs

@@ -0,0 +1,10 @@
+// Copyright 2021 Amazon.com, Inc. or its affiliates. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
+fn main() {
+    // Define a `has_sev` attribute, which is used for conditional
+    // execution of SEV-specific tests and examples.
+    if std::path::Path::new("/dev/sev").exists() {
+        println!("cargo:rustc-cfg=has_sev");
+    }
+}