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/21_image_pad.dts b/tools/binman/test/21_image_pad.dts
new file mode 100644
index 0000000..daf8385f
--- /dev/null
+++ b/tools/binman/test/21_image_pad.dts
@@ -0,0 +1,16 @@
+/dts-v1/;
+
+/ {
+	#address-cells = <1>;
+	#size-cells = <1>;
+
+	binman {
+		pad-byte = <0xff>;
+		u-boot-spl {
+		};
+
+		u-boot {
+			pos = <12>;
+		};
+	};
+};