spiv: image_list: Add more functionality.

* Add lazy counters, so that we can tell total number of images in the
  arg list, including directories

* Add functions to move to the start/end of directory and arg list

* Add functions to return current position in list.

* Make use of the functionality in spiv

TODO: Fix help and write docs.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>