binman: Respect pad-before property of section subentries

Other relevant properties (pad-after, offset, size, align, align-size,
align-end) already work since Pack() sets correct ranges for subentries'
data (.offset, .size variables), but some padding here is necessary to
align the data within this range to match the pad-before property.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/test/166_pad_in_sections.dts b/tools/binman/test/166_pad_in_sections.dts
new file mode 100644
index 0000000..f2b327f
--- /dev/null
+++ b/tools/binman/test/166_pad_in_sections.dts
@@ -0,0 +1,26 @@
+// SPDX-License-Identifier: GPL-2.0+
+
+/dts-v1/;
+
+/ {
+	#address-cells = <1>;
+	#size-cells = <1>;
+
+	binman {
+		pad-byte = <0x26>;
+		section {
+			pad-byte = <0x21>;
+
+			before {
+				type = "u-boot";
+			};
+			u-boot {
+				pad-before = <12>;
+				pad-after = <6>;
+			};
+			after {
+				type = "u-boot";
+			};
+		};
+	};
+};