[Review] Request from 'schubi2' @ 'yast/yast-update/review_140626_checking_file'

Please review the following changes:
  * f689e60 packages
  * c02d0fe checking if the file exists

this looks too confusing for me. simple
```
existing_paths = paths.select { |p| File.exist?(File.join(mounted_root, p)) }
```