binman: Support multithreading for building images

Some images may take a while to build, e.g. if they are large and use slow
compression. Support compiling sections in parallel to speed things up.

Signed-off-by: Simon Glass <sjg@chromium.org>
(fixed to use a separate test file to fix flakiness)
diff --git a/tools/binman/test/202_section_timeout.dts b/tools/binman/test/202_section_timeout.dts
new file mode 100644
index 0000000..1481450
--- /dev/null
+++ b/tools/binman/test/202_section_timeout.dts
@@ -0,0 +1,21 @@
+// SPDX-License-Identifier: GPL-2.0+
+
+/dts-v1/;
+
+/ {
+	#address-cells = <1>;
+	#size-cells = <1>;
+
+	binman {
+		pad-byte = <0x26>;
+		size = <0x28>;
+		section@0 {
+			read-only;
+			size = <0x10>;
+			pad-byte = <0x21>;
+
+			u-boot {
+			};
+		};
+	};
+};