binman: Support optional entries

Support entries which can be optional depending on their contents. This
allows special entry types which appear in the image only when needed.

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

tools/binman/test/262_absent.dts

@@ -0,0 +1,20 @@
+// SPDX-License-Identifier: GPL-2.0+
+
+/dts-v1/;
+
+/ {
+	#address-cells = <1>;
+	#size-cells = <1>;
+
+	binman {
+		u-boot {
+		};
+
+		_testing {
+			set-to-absent;
+		};
+
+		u-boot-img {
+		};
+	};
+};