add support for secondary GPS interface
add support for secondary GPS interface

Useful for navio2 or px4_raspberrypi when attaching a secondary (UART) GPS through one of the USB ports; the default navio2 onboard GPS is running on spi.

Example usage:
gps start -d /dev/spidev0.0 -i spi -e /dev/ttyACM0 -j uart -p ubx

krbeverx/Firmware,krbeverx/Firmware,krbeverx/Firmware,krbeverx/Firmware,acfloria/Firmware,acfloria/Firmware,acfloria/Firmware,acfloria/Firmware,krbeverx/Firmware,krbeverx/Firmware,acfloria/Firmware,acfloria/Firmware,acfloria/Firmware,krbeverx/Firmware