[DeviceConfiguration] General "country" option

[nemesifier]

It makes sense to define "country" in the general section, since it doesn't make much sense to define different radios with different country settings.

Therefore I think a similar configuration:

{
    "type": "DeviceConfiguration",
    "general": {
        "country": "EN"
    },
    "radios": [
        {
            "name": "radio0",
            "phy": "phy0",
            "protocol": "802.11n",
            "channel": 140,
            "channel_width": 20
        },
        {
            "name": "radio1",
            "phy": "phy1",
            "protocol": "802.11n",
            "channel": 132,
            "channel_width": 40
        }
    ]
}

it's better than:

{
    "type": "DeviceConfiguration",
    "radios": [
        {
            "name": "radio0",
            "phy": "phy0",
            "protocol": "802.11n",
            "channel": 140,
            "channel_width": 20,
            "country": "EN"
        },
        {
            "name": "radio1",
            "phy": "phy1",
            "protocol": "802.11n",
            "channel": 132,
            "channel_width": 40,
            "country": "EN"
        }
    ]
}

Because the latter repeats the "country" option for each radio.

PS: this is somewhat similar to #41, in which an interface related setting is included in the "general" section because it affects several parts of the configuration and not a specific one.