settings: implement KeyValueStore (#1659)

* settings: implement KeyValueStore and dependencies

* settings: update KeyValueStore for recent refactoring

* settings: address feedback

libraries/libstratosphere/include/stratosphere/spl/spl_api.hpp

@@ -60,6 +60,12 @@ namespace ams::spl {
         return static_cast<HardwareState>(v);
     }
 
+    inline RetailInteractiveDisplayState GetRetailInteractiveDisplayState() {
+        u64 v;
+        R_ABORT_UNLESS(::ams::spl::GetConfig(std::addressof(v), ::ams::spl::ConfigItem::RetailInteractiveDisplayState));
+        return static_cast<RetailInteractiveDisplayState>(v);
+    }
+
     inline u64 GetDeviceIdLow() {
         u64 v;
         R_ABORT_UNLESS(::ams::spl::GetConfig(std::addressof(v), ::ams::spl::ConfigItem::DeviceId));