nixos/boot: allow kernel.printk override (#34958)

At one point in my configuration I had:
boot.kernel.sysctl = {
      # https://unix.stackexchange.com/questions/13019/description-of-kernel-printk-values
      "kernel.printk" = "4	4	1	7";
};
which triggered:

error: The unique option `boot.kernel.sysctl.kernel.printk' is defined multiple times, in `/home/teto/dotfiles/nixpkgs/mptcp-unstable.nix' and `/home/teto/nixpkgs/nixos/modules/system/boot/kernel.nix'.
(use ‘--show-trace’ to show detailed location information)
Traceback (most recent call last):
  File "/home/teto/nixops/scripts/nixops", line 984, in <module>
    args.op()
  File "/home/teto/nixops/scripts/nixops", line 406, in op_deploy
    max_concurrent_activate=args.max_concurrent_activate)
  File "/home/teto/nixops/nixops/deployment.py", line 1045, in deploy
    self.run_with_notify('deploy', lambda: self._deploy(**kwargs))
  File "/home/teto/nixops/nixops/deployment.py", line 1034, in run_with_notify
    f()
  File "/home/teto/nixops/nixops/deployment.py", line 1045, in <lambda>
    self.run_with_notify('deploy', lambda: self._deploy(**kwargs))
  File "/home/teto/nixops/nixops/deployment.py", line 985, in _deploy
    self.configs_path = self.build_configs(dry_run=dry_run, repair=repair, include=include, exclude=exclude)
  File "/home/teto/nixops/nixops/deployment.py", line 653, in build_configs
    raise Exception("unable to build all machine configurations")
Exception: unable to build all machine configurations

This simple addition allows to override it.