Provide greenboot.target and replace greenboot-healthcheck.service by individual units

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Thanks for this project. I think it's a really nice idea and can be a life-saver to administrate a headless Raspberry Pi.

I wonder there is some potential to simplify the health-check execution. Disclaimer: i'm a big fan of systemd but also not very well versed in either systemd or greenbot and i'm always learning. :)

Provide greenbot.target

This can be RequiredBy=boot-complete.target (if i understand the boot-complete.target correctly.)

Convert the health checks to systemd.service one-shot units

They can be RequiredBy=greenboot.target

I think the fact that greenbot itself has required. and wanted.d is a good indicator that it duplicates systemd logic.

This would also make greenboot-healthcheck.service obsolete.

This would have the following benefits:

• The script doing the logging that healthchecks are run would be a bit obsolte, systemd already provide everything in the journal and tools to visualize the dependency graph (like systemd-analyze critical-path).
• The health checks themselves have appropriate dependencies (like health-checks requiring a network can depend on network-only.target)
• health checks would automatically run in parallel. I think ordering based on file-names does not make sense. systemd dependency and ordering should be used.
• Admins can easily disable specific health-checks (would make provide mechanism for disabling certain default health checks #119 obsolete). This also means if all health-checks that require network are disabled, boot would be way quicker because greenbot wouldn't wait for the network.
• Admins can use all the other fancy systemd functionality to extend/hook-into the integrated health checks
• It would simplify usage of greenbot, because there is only one way that would be needed to be maintained
• It abstracts away the whole shell script thing. Each healthcheck can use whatever runtime it wants. Also no more dependency on shell for boot checks is nice i guess.
• Third party checks can just require/want the greenbot.target and order themselves accordingly how they desire.

Let me know what you think. I would be also happy to try to contribute development efforts, if my freetime allows and the idea is generally accepted.