Config: Flatten structures

Originally, Layer contained a std::map of Sections, which containted a std::map
containing the (key, value) pairs. Here we flattern this structure so that only
one std::map is required, reducing the number of indirections required and
vastly simplifying the code.

Source/Core/Core/Config/MainSettings.cpp

@@ -61,6 +61,7 @@ ConfigInfo<bool> GetInfoForSimulateKonga(u32 channel)
 }
 
 const ConfigInfo<bool> MAIN_WII_SD_CARD{{System::Main, "Core", "WiiSDCard"}, false};
+const ConfigInfo<bool> MAIN_WII_SD_CARD_WRITABLE{{System::Main, "Core", "WiiSDCardWritable"}, true};
 const ConfigInfo<bool> MAIN_WII_KEYBOARD{{System::Main, "Core", "WiiKeyboard"}, false};
 const ConfigInfo<bool> MAIN_WIIMOTE_CONTINUOUS_SCANNING{
     {System::Main, "Core", "WiimoteContinuousScanning"}, false};