Demonstrates a way to differentiate between plain Portenta boards and Portenta Machine Control. The Machine Control has an external pull-up resistor at PB_14, while the plain Portenta boards have a floating PB_14. This sketch injects a high level into the input through the built-in pull-up resistor, floats it, and reads it. Then it injects a low level through the built-in pull-down resistor, floats it, and reads it. With an external pull-up, the pin will always be high. With a floating pin, it will follow the previously injected level from high to low.