rv: add variable to shutdown pi from cloud

[Trek Hopton]

It would be a good feature to be able to remotely shutdown a cloud connected pi running rv. This can be done like in accel-netsender, see this commit

[Trek Hopton]

This was implemented in PR #477, however it was declined in favour of syncreboot:

”Declined pull request
We can now do shutdown using syncreboot. See PR #481. It might be worth however exploring the possibility of having this settable to a long term state, i.e. we would have the device shutdown automatically whenever it's powered on. This could be useful if we have two devices connected to one peripheral port and need to only have one running.

Closed by: Saxon Milton
2022-06-10”

@Alan Noble @Saxon Milton syncreboot exists in utils/cmd/syncreboot and it looks like it’s a program that shuts down or restarts the pi but I don’t know how we were intending to use this in with our rv software. I noticed that the rv makefile installs syncreboot if utils exists on the pi, is the intention to do exec.Command("syncreboot") based on a VidGrind variable that can be set in broadcast off actions?