Support adding kernel commandline options

### Description

#827 added a mechanism to specify additional kernel parameters for a specific image.

Unfortunately this does not allow providing additional parameters via an `override.yaml` or `default.yaml` file because `image` lists are simply concatenated; there is no way to modify attributes of an existing entry like we do for `mounts` and `networks`. There is also no obvious way how this can be made to work for images.

I would like to add a top-level attribute `cmdline` that will be appended in addition to any `cmdline` setting specified for a particular image. That would allow setting things globally, like:

```yaml
cmdline: "vsyscall=emulate"
```

References:
https://github.com/microsoft/WSL/issues/4694
https://gitlab.alpinelinux.org/alpine/aports/-/issues/6927#note_31075