In some cases, new /bin/sh binary doesn't work properly before reboot during a upgrade, and because of that /etc/rc.reboot is not executed and system doesn't reboot. Source /etc/rc.reboot instead of open a new sh session to avoid it happening again in future versions (ticket #3749)
Related issues
Bug #3749: Upgrade from 2.1.4 to 2.2 does not automatically reboot
In some cases, new /bin/sh binary doesn't work properly before reboot during a upgrade, and because of that /etc/rc.reboot is not executed and system doesn't reboot. Source /etc/rc.reboot instead of open a new sh session to avoid it happening again in future versions (ticket #3749)