hostos-update to protect against dual updates

[alexgg]

When a hostOS update is performed, the current root partition link is updated to point to the new partition. If the system is not rebooted then, the next hostOS update will overwrite the current running partition, and this should not be allowed.

The update script should check whether the root partition link points to the inactive partition, and switch partitions before going ahead with the update to avoid overwriting both partitions without a reboot.