expo: Create a struct for generic text attributes

In preparation for adding more text types, refactor the common fields
into a new structure. This will allow common code to be used.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/boot/cedit.c b/boot/cedit.c
index 792ab6d..0f7a865 100644
--- a/boot/cedit.c
+++ b/boot/cedit.c
@@ -271,7 +271,7 @@
 	if (!txt)
 		return log_msg_ret("txt", -ENOENT);
 
-	str = expo_get_str(scn->expo, txt->str_id);
+	str = expo_get_str(scn->expo, txt->gen.str_id);
 	if (!str)
 		return log_msg_ret("str", -ENOENT);
 	*strp = str;