Various fixes and improvements (#177)

- Remove custom `ESP_TARGET` and use `ESP_PLATFORM` everywhere
- Add `Loader` service functionality to `tt::app::` namespace
- Make `Loader` `PubSub` usable by exposing the messages
- Add board type to crash log
- Don't show SD card in Files app when it's not mounted
- Set default SPI frequency for SD cards
- Move TT_VERSION to scope that works for sim too
- Log Tactility version and board on boot
- Rename "Yellow Board" to "CYD 2432S024C"

Tactility/Source/app/App.h

@@ -3,6 +3,7 @@
 #include "AppContext.h"
 #include "Bundle.h"
 #include <Mutex.h>
+#include <string>
 
 // Forward declarations
 typedef struct _lv_obj_t lv_obj_t;
@@ -75,4 +76,20 @@ public:
 template<typename T>
 std::shared_ptr<App> create() { return std::shared_ptr<T>(new T); }
 
+/**
+ * @brief Start an app
+ * @param[in] id application name or id
+ * @param[in] parameters optional parameters to pass onto the application
+ */
+void start(const std::string& id, std::shared_ptr<const Bundle> _Nullable parameters = nullptr);
+
+/** @brief Stop the currently showing app. Show the previous app if any app was still running. */
+void stop();
+
+/** @return the currently running app context (it is only ever null before the splash screen is shown) */
+std::shared_ptr<app::AppContext> _Nullable getCurrentAppContext();
+
+/** @return the currently running app (it is only ever null before the splash screen is shown) */
+std::shared_ptr<app::App> _Nullable getCurrentApp();
+
 }