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-binman.dts

@@ -0,0 +1,24 @@
+// SPDX-License-Identifier: GPL-2.0+
+
+/dts-v1/;
+
+/ {
+	#address-cells = <1>;
+	#size-cells = <1>;
+
+	binman {
+		filename = "sandbox.img";
+
+		pre-load {
+			 content = <&image>;
+			 algo-name = "sha256,rsa2048";
+			 key-name = "dev.key";
+			 header-size = <4096>;
+			 version = <1>;
+		};
+
+		image: blob-ext {
+			 filename = "test.fit";
+		};
+	};
+};