Do not throw warnings when /tmp/custom_mounts.list does not exist.

When no device for an overlay can be found, /tmp/custom_mounts.list
won't be created and will produce warnings while booting:

  sort: /tmp/custom_mounts.list: No such file or directory
  rm: can't remove '/tmp/custom_mounts.list': No such file or directory

Properly handle this case by calling rm with the -f option and calling
sort only when the file exists.