support running Talos dashboard on e.g. `ttyS0`

Add new kernel arg, e.g. `talos.dashboard.console=ttyS0`.

If specified, it does following chanages:

* it stops switching VTTYs in `services/dashboard.go`
* it runs dashboard process on `/dev/` + value of the arg as stdin/stdout. 

We should pre-validate that the name starts with `tty`. 

We don't handle conflict of e.g. setting `console=ttyS0`, it's up to the user to decide what they are doing.