Add a way to disconnect the power to the board through GPIO

[spiccinini]

The idea is to provide a way to cut the power to the LibreRouter.

[xcancerberox]

Like a self reset? or like suicide?

[spiccinini]

Like a self reset? or like suicide?

That functionality is already present through the watchdog that is also available at a GPIO.

This idea is to provide a way for an external board to use a GPIO ot cut the power of the board for longer periods of time. For example when connected to a solar panel and batteries in the night the LR can be turned on like for 2 minute each 10 minutes saving 80% of the battery power (even detecting if there is a station connected and keeping it on longer periods of time).

Ideally the voltage should be provided from the external source and the GPIO work from 3.3 to 24V.

[xcancerberox]

Ok, so the GPIO is an external GPIO. We can add there a PMOS transistor like: with the same transistors that we are using for POE.