lbximg: Add an option to control the output filenames.

lbximg: Add an option to control the output filenames.

This will be needed for postmortem analysis of test cases that extract
multiple images, since otherwise there is no easy way to prevent the
tool from overwriting the earlier outputs.

This was clearly intended to be a user setting from the start but for
some reason it was never wired up to a command-line option.