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>

test/vboot/sandbox-kernel.dts

@@ -0,0 +1,7 @@
+/dts-v1/;
+
+/ {
+	model = "Sandbox Verified Boot Test";
+	compatible = "sandbox";
+
+};