GPS.cpp passes GPS_RX_PIN as the MCU's RX pin and GPS_TX_PIN as its TX
pin. Nine variants documented the opposite, which reads as if the pins
were swapped on working hardware (see #11584).
Comment-only change; no pin assignment is touched.
* Just set LED_BUILTIN universally to -1, as we don't use it.
* LUD_BUILTIN workarounds
* Squash the LED_BUILTINs that sneaked in
* Don't kill valid pin derfine