write ubuntu_command file atomically

[Barry Warsaw]

There's a small window of opportunity for mischief. Let's say something goes wrong while we're writing the ubuntu_command file, and yet the device still gets rebooted. recovery will see the incomplete ubuntu_command file and will apply an incomplete set of commands, which could brick the phone.

The solution is rather easy: write the ubuntu_command file atomically. If a failure occurs during write, then recovery won't see an update. Only once the file is successfully written do we toggle the file in place and reboot.