binman: Allow disabling expanding an entry

At present there is a command-line flag to disable substitution of expanded
entries. Add an option to the entry node as well, so it can be controlled
at the node level.

Add a test to cover this. Fix up the comment to the checkSymbols() function
it uses, while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>

tools/binman/test/197_symbols_expand.dts

@@ -0,0 +1,23 @@
+// SPDX-License-Identifier: GPL-2.0+
+
+/dts-v1/;
+
+/ {
+	#address-cells = <1>;
+	#size-cells = <1>;
+
+	binman {
+		pad-byte = <0xff>;
+		u-boot-spl {
+		};
+
+		u-boot {
+			offset = <0x38>;
+			no-expanded;
+		};
+
+		u-boot-spl2 {
+			type = "u-boot-spl";
+		};
+	};
+};