squash: squash systemd binary and udevadm
    
    systemd binary and udevadm are not needed to be outside the squash
    image. Some binaries are kept outside because they are required before
    mounting the image, or after umounting the image (when switching root),
    or they may block umounting the image. But we are using lazy umounting,
    so actually nothing will block the umount.
    
    Keep more binaries outside the squash image won't hurt but cost extra
    memories, the idea of squash image is to save memory usage.
    
    So, there is no reason to keep udevadm outside, that should be a debug
    left over. For systemd binary, it's running when switch root happens,
    But we have lazy umounted the image and overlay, once systemd process
    exec the new systemd in new root, everything will be cleared by kernel.
    
    Also tidy up the comment make it less confussing.
    
    Signed-off-by: Kairui Song <kasong@redhat.com>
    (cherry picked from commit e1e1f6e8e6747d8f32c065e267e0a57587818c9e)
    
    Resolves: #1691705
    
    patch_name: 0018.patch
    present_in_specfile: true
    location_in_specfile: 18
    squash_commits: true