external_gpio: Add a warning and disable by default

Signed-off-by: Alistair Francis <alistair23@gmail.com>
external_gpio: Update the python script

Signed-off-by: Alistair Francis <alistair23@gmail.com>
external_gpio: Add a timer to fake reads

Signed-off-by: Alistair Francis <alistair23@gmail.com>
external_gpio: Correct checking for changes

Signed-off-by: Alistair Francis <alistair23@gmail.com>