AP_GPS: add proxy driver for GNSS modules handled by PX4 firmware
AP_GPS: enable GNSS modules handled by PX4 firmware via GPS_TYPE

GNSS modules handled by PX4 drivers are not auto-detectable, some are not even
connected to a UART port. The activation is therefore controlled by GPS_TYPE
only. Baud rate and port settings (if applicable) have to be handled by the PX4
firmware.
AP_GPS: added PX4EXPERIMENTAL to GPS type drop down
AP_GPS: fixed build on non-PX4 platforms