A board only needs to define:
#define RC_SERIAL_PORT "/dev/ttyS4"
Then it can optionally define one or more of the following:
#define RC_SERIAL_SWAP_RXTX
#define RC_SERIAL_SINGLEWIRE
#define RC_INVERT_INPUT(_invert_true) px4_arch_gpiowrite(GPIO_SBUS_INV, _invert_true)