expo: Use an abuf to hold strings

It is more convenient to put strings in an abuf so they can easily be
resized. Adjust the struct accordingly.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/expo.h b/include/expo.h
index 84dc77f..b6de031 100644
--- a/include/expo.h
+++ b/include/expo.h
@@ -134,12 +134,12 @@
  * struct expo_string - a string that can be used in an expo
  *
  * @id: ID number of the string
- * @str: String
+ * @buf: String (contains nul terminator)
  * @sibling: Node to link this object to its siblings
  */
 struct expo_string {
 	uint id;
-	const char *str;
+	struct abuf buf;
 	struct list_head sibling;
 };