Use config files to store whether SDMC is enabled or not

Before, it used to use whether the directory actually existed. As a result, .citra-emu/sdmc was never auto-created (something quite confusing to me until I read through the logs).

src/citra/config.h

@@ -16,6 +16,7 @@ class Config {
 
     bool LoadINI(INIReader* config, const char* location, const std::string& default_contents="", bool retry=true);
     void ReadControls();
+    void ReadData();
 public:
     Config();
     ~Config();