Upgrade method of the device.
none - No operation was performed.
reboot - The upgrade method of this device is Reboot.
sequenceReboot - The upgrade method of this device is SequenceReboot.
issuReboot - The upgrade method of this device is IssuReboot.
serviceUpgrade - The upgrade method of this device is ServiceReboot.
fileUpgrade - The upgrade method of this device is FileReboot.
incompatibleUpgrade - The upgrade method of this device is IncompatibleUpgrade.