binman: Add x86 support for starting TPL

Sometimes we want to include TPL for x86 platforms, such as when we want
to select between different SPL images (e.g. for Chrome OS verified boot).
Add support for this.

Signed-off-by: Simon Glass <sjg@chromium.org>

tools/binman/test/81_x86-start16-tpl.dts

@@ -0,0 +1,14 @@
+// SPDX-License-Identifier: GPL-2.0+
+/dts-v1/;
+
+/ {
+	#address-cells = <1>;
+	#size-cells = <1>;
+
+	binman {
+		size = <16>;
+
+		x86-start16-tpl {
+		};
+	};
+};