Automatically set X2 stepper pins

.travis.yml

Marlin/Configuration_adv.h

Marlin/example_configurations/Felix/Configuration_adv.h

Marlin/example_configurations/Hephestos/Configuration_adv.h

...in/example_configurations/Hephestos_2/Configuration_adv.h

Marlin/example_configurations/K8200/Configuration_adv.h

Marlin/example_configurations/RigidBot/Configuration_adv.h

Marlin/example_configurations/SCARA/Configuration_adv.h

Marlin/example_configurations/TAZ4/Configuration_adv.h

Marlin/example_configurations/WITBOX/Configuration_adv.h

...n/example_configurations/delta/biv2.5/Configuration_adv.h

.../example_configurations/delta/generic/Configuration_adv.h

...mple_configurations/delta/kossel_mini/Configuration_adv.h

...ample_configurations/delta/kossel_pro/Configuration_adv.h

...xample_configurations/delta/kossel_xl/Configuration_adv.h

Marlin/example_configurations/makibox/Configuration_adv.h

.../example_configurations/tvrrug/Round2/Configuration_adv.h

Marlin/macros.h

Marlin/pins.h

Marlin/stepper.cpp