IMX6 Reset Button

That is exposed to linux as a GPIO key. You can see it in the device-tree here:

 arch/arm/boot/dts/imx6qdl-cubox-i.dtsi 

 
    pinctrl_gpio_key: gpio-key {
                        fsl,pins = <
                                MX6QDL_PAD_EIM_DA8__GPIO3_IO08  0x17059
                        >;
                };

If you want you can customize this to look like a custom button. http://lxr.free-electrons.com/source/Documentation/devicetree/bindings/input/gpio-keys.txt

Then you would need to write a userspace daemon to listen for the key event and then perform whatever action you wanted depending on the length it was held down for.