-Given a filename (i.e., fw_cfg item name), starting pointer, and size,
-completely replace the configuration item referenced by the given item
-name with the new given blob. If an existing blob is found, its
-callback information is removed, and a pointer to the old data is
-returned to allow the caller to free it, helping avoid memory leaks.
-If a configuration item does not already exist under the given item
-name, a new item will be created as with fw_cfg_add_file(), and NULL
-is returned to the caller. In any case, the data referenced by the
-starting pointer is only linked, NOT copied, into the fw_cfg data
-structure.
+To check the result, read the "control" field:
+ error bit set -> something went wrong.
+ all bits cleared -> transfer finished successfully.
+ otherwise -> transfer still in progress (doesn't happen
+ today due to implementation not being async,
+ but may in the future).