binman: Support compression of sections

With the previous changes, it is now possible to compress entire
sections. Add some tests to check that compression works correctly,
including updating the metadata.

Also update the documentation.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/test/182_compress_image.dts b/tools/binman/test/182_compress_image.dts
new file mode 100644
index 0000000..4176b7f
--- /dev/null
+++ b/tools/binman/test/182_compress_image.dts
@@ -0,0 +1,14 @@
+// SPDX-License-Identifier: GPL-2.0+
+/dts-v1/;
+
+/ {
+	binman {
+		compress = "lz4";
+		blob {
+			filename = "compress";
+		};
+
+		u-boot {
+		};
+	};
+};