binman: Allow reading entries from a subnode

Some images may have multiple copies of the same thing, e.g. two versions
of the read/write U-Boots. It is necessary to read data from one or other
of these under selection of the verified-boot logic. Add a function to
select the subnode to use.

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

include/binman.h

@@ -70,6 +70,20 @@ int binman_entry_find(const char *name, struct binman_entry *entry);
  */
 ofnode binman_section_find_node(const char *name);
 
+/**
+ * binman_select_subnode() - Select a subnode to use to find entries
+ *
+ * Normally binman selects the top-level node for future entry requests, such as
+ * binman_entry_find(). This function allows a subnode to be chosen instead.
+ *
+ * @name: Name of subnode, typically a section. This must be in the top-level
+ *	binman node
+ * @return 0 if OK, -EINVAL if there is no /binman node, -ECHILD if multiple
+ *	images are being used but the first image is not available, -ENOENT if
+ *	the requested subnode cannot be found
+ */
+int binman_select_subnode(const char *name);
+
 /**
  * binman_init() - Set up the binman symbol information
  *