binman: Support adding a U-Boot environment

In some cases it is useful to include a U-Boot environment region in an
image. This allows the board to start up with an environment ready to go.

Add a new entry type for this. The input is a text file containing the
environment entries, one per line, in the format:

   var=value

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
diff --git a/tools/binman/test/174_env.dts b/tools/binman/test/174_env.dts
new file mode 100644
index 0000000..d1393d2
--- /dev/null
+++ b/tools/binman/test/174_env.dts
@@ -0,0 +1,20 @@
+// SPDX-License-Identifier: GPL-2.0+
+
+/dts-v1/;
+
+/ {
+	#address-cells = <1>;
+	#size-cells = <1>;
+
+	binman {
+		u-boot {
+		};
+		u-boot-env {
+			filename = "env.txt";
+			size = <0x18>;
+			fill-byte = [ff];
+		};
+		u-boot-nodtb {
+		};
+	};
+};