Added support for not using ser2net. Closes #4

[wwahammy]

Some people won't want to use ser2net to connect to their DUT. Instead they'll want to connect via a local serial port. This commit makes that possible by doing two things:

1. set connection_type in your board configuration to anything other than ser2net. (ser2net is still the default)
2. in conn_cmd: use the command line you'd normally connect to the serial port of your DUT. For example, I used "screen /dev/ttyUSB0 115200"

To clarify, the relevant parts of my my boardfarm_config_example.json are as follows:

{
    "board01": {
        "board_type": "db120",
        "conn_cmd": "screen /dev/ttyUSB0 115200",
        "lan_device": "rpi3",
        "location": "austin",
        "powerport": "8",
        "wan_device": "rpi2",
        "notes": "Old board",
        "connection_type": "anything" 
    }
}

I don't think this is ideal but it should get us past the problem of local serial ports not working.

I don't have the ability to test this yet so I'm not merging until someone else tests this :)

[wwahammy]

Closing because I have a better solution coming soon

[wwahammy]

An updated solution to this is at #22