Add quantum/rgblight_post_config.h and quantum/split_common/post_config.h using POST_CONFIG_H variable of build_keyboard.mk.
quantum/rgblight_post_config.h additionally defines RGBLIGHT_SPLIT if RGBLED_SPIT is defined.
quantum/split_common/post_config.h defines RGBLIGHT_SPLIT additionally when master-slave communication is I2C.