add 'find' function for the tree

That will be useful to search for a specific node name.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
diff --git a/tree.h b/tree.h
index 57c70f8..7be3fc4 100644
--- a/tree.h
+++ b/tree.h
@@ -42,4 +42,6 @@
 
 extern struct tree *tree_load(const char *path, tree_filter_t filter);
 
+extern struct tree *tree_find(struct tree *tree, const char *name);
+
 extern int tree_for_each(struct tree *tree, tree_cb_t cb, void *data);