This way you can supply a `boot.iso`, `boot.img` or `boot.qcow2` file. The value of the `BOOT`variable will be ignored in this case.
You can supply a `boot.iso`, `boot.img` or `boot.qcow2` file by replacing the example path `./example.iso` with the filename of your desired image. The value of `BOOT` will be ignored in this case.