binman: Add support for calling mkimage

As a first step to integrating mkimage into binman, add a new entry type
that feeds data into mkimage for processing and incorporates that output
into the image.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/test/156_mkimage.dts b/tools/binman/test/156_mkimage.dts
new file mode 100644
index 0000000..933b131
--- /dev/null
+++ b/tools/binman/test/156_mkimage.dts
@@ -0,0 +1,23 @@
+// SPDX-License-Identifier: GPL-2.0+
+
+/dts-v1/;
+
+/ {
+	#address-cells = <1>;
+	#size-cells = <1>;
+
+	binman {
+		size = <0x80>;
+
+		mkimage {
+			args = "-n test -T script";
+
+			u-boot-spl {
+			};
+
+			_testing {
+				return-contents-later;
+			};
+		};
+	};
+};