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>
diff --git a/tools/binman/entry.py b/tools/binman/entry.py
index 1cfa024..ac25986 100644
--- a/tools/binman/entry.py
+++ b/tools/binman/entry.py
@@ -164,7 +164,8 @@
         if obj and expanded:
             # Check whether to use the expanded entry
             new_etype = etype + '-expanded'
-            if obj.UseExpanded(node, etype, new_etype):
+            can_expand = not fdt_util.GetBool(node, 'no-expanded')
+            if can_expand and obj.UseExpanded(node, etype, new_etype):
                 etype = new_etype
             else:
                 obj = None