Bone shutdown signal


I'm using a beaglebone in a battery-powered application, where I'm
supplying power from a cape to the bone through the header. Sometimes
my device isn't using the bone and I want to turn it off to save
power, and I'm looking for a way to do that cleanly.

The issue is that I want to turn the bone off properly using the
shutdown command, but I haven't found any way to indicate to the cape
that the shutdown has completed. How can I determine when it's safe
to cut the power?