binman: Support writing symbols inside a mkimage image

Add support for writing symbols and determining the assumed position of
binaries inside a mkimage image. This is useful as an example for other
entry types which might want to do the same thing.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/test/290_mkimage_sym.dts b/tools/binman/test/290_mkimage_sym.dts
new file mode 100644
index 0000000..2dfd286
--- /dev/null
+++ b/tools/binman/test/290_mkimage_sym.dts
@@ -0,0 +1,27 @@
+// SPDX-License-Identifier: GPL-2.0+
+
+/dts-v1/;
+
+/ {
+	#address-cells = <1>;
+	#size-cells = <1>;
+
+	binman {
+		u-boot-dtb {
+		};
+
+		mkimage {
+			args = "-n test -T script";
+
+			u-boot-spl {
+			};
+
+			u-boot-spl2 {
+				type = "u-boot-spl";
+			};
+		};
+
+		u-boot {
+		};
+	};
+};