Check a flag file to decide whether to boot into recovery

With these changes, u-boot checks for the existence of /persist/reboot-recovery to decide when to reboot into recovery (in addition to holding the volume up key). By including a blank persist partition in the device tarball, we ensure that the flag file is deleted when an update completes. Otherwise, we can delete the flag file early in Jumpdrive's init.

This isn't a good option, but it will do until we find something else.