binman: Add basic entry types for U-Boot

Add entries to support some standard U-Boot binaries, such as u-boot.bin,
u-boot.dtb, etc. Also add some tests for these.

Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/tools/binman/test/06_dual_image.dts b/tools/binman/test/06_dual_image.dts
new file mode 100644
index 0000000..78be16f
--- /dev/null
+++ b/tools/binman/test/06_dual_image.dts
@@ -0,0 +1,22 @@
+/dts-v1/;
+
+/ {
+	#address-cells = <1>;
+	#size-cells = <1>;
+
+	binman {
+		multiple-images;
+		image1 {
+			u-boot {
+			};
+		};
+
+		image2 {
+			pad-before = <3>;
+			pad-after = <5>;
+
+			u-boot {
+			};
+		};
+	};
+};