Power signal control definitions for other power supplies

To turn on the power supply by default PS_ON need to be grounded.
To turn it off PS_ON send 5V. This is given by the ATX standard.
This logic is reversed with an XBox 360 203Watts power supply, PS_ON have to send 5V to turn on the power supply.
I assumed other signal control can exist so I let the possibility to manage PS_ON.
I'm not sure about where in the code PS_ON have to be initialized at startup.

Tested with a RRD RAMPS 1.4, Repetier and an XBox 360 203Watts power supply which now understand a M80 or M81 command properly.