085a8ef checksum: Checksum each image only once

Authored and Committed by lsedlar 6 years ago
    checksum: Checksum each image only once
    
    There is no point in reading the same image multiple times. This happens
    for at least source ISOs.
    
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>