Yo can use the JsonConvert class to read your json:
var deviceData = JsonConvert.DeserializeObject<DeviceData>("...yourJson..");
Your classes should look like this:
public class DeviceData {
    public DeviceList Devices { get; set; }
}
public class DeviceList {
    [JsonProperty(PropertyName = "AppleTV2,1")]
    public Device AppleTV21 { get; set; }
    [JsonProperty(PropertyName = "AppleTV3,1")]
    public Device AppleTV31 { get; set; }
    // continue ...
}
public class Device {
    public string Name { get; set; }
    public string BoardConfig { get; set; }
    public string Platform { get; set; }
    public string Cpid { get; set; }
    public string Bdid { get; set; }
    public Firmware[] Firmwares { get; set; }
}
public class Firmware {
    public string Version { get; set; }
    public string BuildId { get; set; }
    public string Sha1Sum { get; set; }
    public string Md5Sum { get; set; }
    public int Size { get; set; }
    public DateTime ReleaseDate { get; set; }
    public DateTime UploadDate { get; set; }
    public string Url { get; set; }
    public bool Signed { get; set; }
    public string Filename { get; set; }
}