binman: Support ELF files for U-Boot and SPL

For sandbox we want to put ELF files in the image since that is what we
need to execute. Add support for this.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/test/97_elf_strip.dts b/tools/binman/test/97_elf_strip.dts
new file mode 100644
index 0000000..6f3c66f
--- /dev/null
+++ b/tools/binman/test/97_elf_strip.dts
@@ -0,0 +1,15 @@
+// SPDX-License-Identifier: GPL-2.0+
+/dts-v1/;
+
+/ {
+	#address-cells = <1>;
+	#size-cells = <1>;
+
+	binman {
+		u-boot-elf {
+			strip;
+		};
+		u-boot-spl-elf {
+		};
+	};
+};