abuf: Add a function to copy a buffer

It is useful to be able to copy an abuf, to allow changes while
preserving the original. Add a function for this.

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

include/abuf.h

@@ -111,6 +111,17 @@ bool abuf_realloc(struct abuf *abuf, size_t new_size);
  */
 bool abuf_realloc_inc(struct abuf *abuf, size_t inc);
 
+/**
+ * abuf_copy() - Make a copy of an abuf
+ *
+ * Creates an allocated copy of @old in @new
+ *
+ * @old: abuf to copy
+ * @new: new abuf to hold the copy (inited by this function)
+ * Return: true if OK, false if out of memory
+ */
+bool abuf_copy(const struct abuf *old, struct abuf *new);
+
 /**
  * abuf_uninit_move() - Return the allocated contents and uninit the abuf
  *