[FL-872] Furi, API-HAL, App-Loader cleanup and improvements (#334)

* Furi: replace obsolete furiac_exit with osThreadExit, drop obsolete apis and test. Rename systemd to flipper and move to separate file, cleanup. ApiHal: Rename timebase to os and move freertos hooks there, move insomnia api to power module.
* Furi: new thread helper
* Furi: cleanup thread documentation
* Flipper, AppLoader: update to use FuriThread. Update tasks signatures to match FuriThreadCallback signature.
* F4: rename API_HAL_TIMEBASE_DEBUG to API_HAL_OS_DEBUG
* Applications: rename FuriApplication to FlipperApplication, use FuriThreadCallback signature for apps.
* C++ app template sample, new exit method

applications/sd-card-test/sd-card-test.cpp

@@ -904,7 +904,8 @@ void SdTest::render(Canvas* canvas) {
 }
 
 // app enter function
-extern "C" void sd_card_test(void* p) {
+extern "C" int32_t sd_card_test(void* p) {
     SdTest* app = new SdTest();
     app->run();
+    return 0;
 }