Report uptime in OS checks

[zikada]

The attestation result currently lacks any indication that the device was rebooted, important because reboots reverify the device with verified boot.

Please consider adding the system uptime to one of the attested properties in the OS checks part.

Maybe it would be more useful with #53.

Edit: The attestation server could use this to suggest the user as a general practice to regularly reboot their device, if it sees a high uptime. Some users only reboot for OTA updates, and then some users miss the OTA update notification at all. The server could suggest rebooting and checking for OTA updates if it sees an extremely high uptime.

Thanks