Readded the [Install] section, and added 'ConditionKernelCommandLine=|bootchooser.active' to make this service trigger only if barebox's bootchooser framework passed the correct argument into the kernel command line.

This replaces the previous service that marked the system as good when booting up with one that has the following:

- Starts after sshd, login prompt on serial console, and NetworkManager have all started up.
- Requires those three services to be up, not simply wanting them.
- If any of them fail to start, the rauc-mark-good.service fails with the result dependency and the rauc-failure.service included by this commit is triggered, and immediately reboots the system.
- The [Install] section is no longer required due to the service being pulled along by its own dependencies.