Make the GPIO ports configurable

[white-gecko]

When I connected the brachio graph to a raspberry pi 1 it accidentally rebooted while the arms were attached. During the boot process the shoulder servo turned rapidly around. I think this is caused by the fact that the GPIO port 14 is also used for the serial console in a headless setup: https://pinout.xyz/pinout/pin8_gpio14#

A simple solution would be to make the GPIO ports configurable so people can change the shoulder port if they experience similar behavior.