abuf: Provide a constant buffer

Add a new initialiser which can accept a constant pointer.

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

include/abuf.h

@@ -157,6 +157,19 @@ void abuf_init_move(struct abuf *abuf, void *data, size_t size);
  */
 void abuf_init_set(struct abuf *abuf, void *data, size_t size);
 
+/**
+ * abuf_init_const() - Set up a new const abuf
+ *
+ * Inits a new abuf and sets up its (unallocated) data. The only current
+ * difference between this and abuf_init_set() is the 'data' parameter is a
+ * const pointer. At some point a flag could be used to indicate const-ness.
+ *
+ * @abuf: abuf to set up
+ * @data: New contents of abuf
+ * @size: New size of abuf
+ */
+void abuf_init_const(struct abuf *abuf, const void *data, size_t size);
+
 /**
  * abuf_uninit() - Free any memory used by an abuf
  *