Is it possible that 'poweroff' actually reboots the box? Poweroff on an embedded system can be tricky. If the kernel doesn't know how to switch off the box, it can only reboot, when it's at the end of the shutdown procedure. Some embedded boxes cannot shutdown at all (there is no logic which can cut the power), and emulate it by letting the kernel enter an endless loop.
You can try a 'init 0' or 'halt'. If that fails, and the kernel has 'sysreq' support, you can try
echo 1 > /proc/sys/kernel/sysrq
echo o > /proc/sysrq-trigger
(You can see if the kernel supports it by the existence of /proc/sysrq-trigger.
If that fails you'll have to find out how the firmware shuts down the box. Possibly some script. Try to press <tab><tab> to get a list of executables. Maybe some name is descriptive.