binman: Place Intel descriptor at image start

The Intel descriptor must always appear at the start of an (x86) image,
so it is supposed to position itself there always. However there is no
explicit test for this. Add one and fix a bug introduced by the recent
change to adjust Entry to read the node in a separate call.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/test/141_descriptor_offset.dts b/tools/binman/test/141_descriptor_offset.dts
new file mode 100644
index 0000000..f9bff01
--- /dev/null
+++ b/tools/binman/test/141_descriptor_offset.dts
@@ -0,0 +1,20 @@
+// SPDX-License-Identifier: GPL-2.0+
+
+/dts-v1/;
+
+/ {
+	#address-cells = <1>;
+	#size-cells = <1>;
+
+	binman {
+		sort-by-offset;
+		end-at-4gb;
+		size = <0x800000>;
+		u-boot {
+			offset = <0xffff0000>;
+		};
+		intel-descriptor {
+			filename = "descriptor.bin";
+		};
+	};
+};