test: py: vboot: add test for global image signature

Adds test units for the pre-load header signature.

Signed-off-by: Philippe Reynes <philippe.reynes@softathome.com>

test/py/tests/vboot/sandbox-u-boot-global.dts

@@ -0,0 +1,27 @@
+// SPDX-License-Identifier: GPL-2.0+
+
+/dts-v1/;
+
+/ {
+	model = "Sandbox Verified Boot Test";
+	compatible = "sandbox";
+
+	binman {
+	};
+
+	reset@0 {
+		compatible = "sandbox,reset";
+	};
+
+	image {
+		pre-load {
+			sig {
+				algo-name = "sha256,rsa2048";
+				signature-size = <256>;
+				mandatory = "yes";
+
+				key-name = "dev";
+			};
+		};
+	};
+};