Support u-blox GPS I2C interface

[JayKickliter]

Overview

This PR allows the user to configure the SX1302 packet forwarder to use a u-blox GPS with the I2C interface instead of just TTY.

Currently, one signals to the forwarder that a GPS is available by adding the following line to the gateway_conf section in global_config.json:

        "gps_tty_path": "/dev/tty<N>",

Alternatively, the user can now specify a different (and mutually exclusive) parameter:

        "gps_i2c_path": "/dev/i2c-<N>",

Remaining

• [x] BLOCKING figure out why this line frequently prints WARNING: n_queued value 0x0 appears to be glitchy (0 should not be possible)
• [x] BLOCKING add new data to json protocol as was done with the SX1301 packet forwarder
• [x] QUESTION rip out NMEA parsing? We did with the SX1301 packet forwarder, but the SX1302 version actively maintained and we may want to upstream

For Reviewers

The most important changes in this PR hidden by default and easily overlooked when skimming the diff tab above: libloragw/src/loragw_gps.c and packet_forwarder/src/lora_pkt_fwd.c

[fvasquez]

QUESTION rip out NMEA parsing? We did with the SX1301 packet forwarder, but the SX1302 version actively maintained and we may want to upstream

I say keep the NMEA parsing for those folks who may not have a u-blox GPS module.