Add a "master" or "batt" button, which allows you to activate/deactivate the battery of vehicles (planes, cars, etc.)

What are you wanting to use this for?

It just to add more realistic thing

No no, specifically what are you looking to do with it? What realistic functions?

I would like a button/switch in the panel which allows to activate the main current of the vehicle (master switch), for example we will not be able to start the engine / lamps without the master being on

The newest version of IVV even uses this with the IVLabs ACIVE! The on-board computer (hidden seat driver) is able to activate a drivers seat control lockout in the event that the interior cabin is compromised. (You can hide all the buttons from the driver's seat)

Ah, in that case what you can do is make a customVariable, then, if it's 0, you turn off any engine magnetos or starters that show up. This is something that can happen with current custom panels and variableModifiers, and I don't see a need to add internally because of this.