Create `/run/reboot-required` by default

[cgwalters]

https://github.com/kubereboot/kured expects this and I can't imagine there's downsides to us just doing this by default and avoiding people needing to create bridging manually.

[vlaborie]

Hi,

I use Kured on Kubernetes with bootc node, for info it's work just by setting the reboot-sentinel param to /run/ostree/staged-deployment, eg. with Helm values:

values:
  configuration:
    rebootSentinel: /run/ostree/staged-deployment

But create /run/reboot-required can be usefull for a working Kured config by default.