Add verified boot information and test

Add a description of how to implement verified boot using signed FIT images,
and a simple test which verifies operation on sandbox.

The test signs a FIT image and verifies it, then signs a FIT configuration
and verifies it. Then it corrupts the signature to check that this is
detected.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/test/vboot/sign-images.its b/test/vboot/sign-images.its
new file mode 100644
index 0000000..f69326a
--- /dev/null
+++ b/test/vboot/sign-images.its
@@ -0,0 +1,42 @@
+/dts-v1/;
+
+/ {
+	description = "Chrome OS kernel image with one or more FDT blobs";
+	#address-cells = <1>;
+
+	images {
+		kernel@1 {
+			data = /incbin/("test-kernel.bin");
+			type = "kernel_noload";
+			arch = "sandbox";
+			os = "linux";
+			compression = "none";
+			load = <0x4>;
+			entry = <0x8>;
+			kernel-version = <1>;
+			signature@1 {
+				algo = "sha1,rsa2048";
+				key-name-hint = "dev";
+			};
+		};
+		fdt@1 {
+			description = "snow";
+			data = /incbin/("sandbox-kernel.dtb");
+			type = "flat_dt";
+			arch = "sandbox";
+			compression = "none";
+			fdt-version = <1>;
+			signature@1 {
+				algo = "sha1,rsa2048";
+				key-name-hint = "dev";
+			};
+		};
+	};
+	configurations {
+		default = "conf@1";
+		conf@1 {
+			kernel = "kernel@1";
+			fdt = "fdt@1";
+		};
+	};
+};