CFG:U: Implemented the GetCountryCodeID and GetCountryCodeString

I couldn't find a way to retrieve all the country code strings, and the ones in there need to be checked, probably using a real 3DS.
Does the 3DS send those strings with a NULL terminator? that's another thing that has to be checked on a real 3DS.
If you do `std::string country_code_string(cmd_buffer[i]);`, later on you can just do `if (country_code_string == code) {...}` without using `strcmp`. A little more readable + modern.