I'm setting up an embedded device running Debian Jessie to shut down when a UPS experiences a power failure. Unfortunately, the device's bootloader restarts the OS whenever it quits, so shutdown/halt commands are equivalent to restart.

I think my best option is to kill as many tasks as possible and then wait for the power to cut. systemctl isolate emergency.target seems close to what I want, except it leaves disks mounted. Is there a way to get to final.target without an actual shutdown?