[simu] Refactor simu startup and shutdown handling (for 2.3 branch):

* Use pwrCheck() as primary control point for shutdown, centralizes thread control in simpgmspace module;
 * Remove main_thread_running in favor of separate variables for startup mode and shutdown flag;
 * Don't start a thread for ARM startup, just run simuMain() directly (the thread just exits anyway);
 * Refactor SIMU_SLEEP() macro, and remove all unnecessary uses of it, also remove SIMU_SLEEP_NORET;
 * Add startType param to opentxStart() to control splash and startup checks;
 * Fixes backlight never turning off in simulator with "SWITCH" power button type (pwrPressed() was always true);
 * Fixes simu shutdown when model checklist is displayed at startup;
 * Adds stubs to possibly simulate a "soft" power button in the future.

radio/src/io/bootloader_flash.cpp

@@ -61,7 +61,8 @@ void bootloaderFlash(const char * filename)
       flashWrite(CONVERT_UINT_PTR(FIRMWARE_ADDRESS+i+j), (uint32_t *)(buffer+j));
     }
     drawProgressBar(STR_WRITING, i, BOOTLOADER_SIZE);
-    SIMU_SLEEP(30/*ms*/);
+    if (!SIMU_SLEEP(30/*ms*/))
+      break;
   }
 
   if (unlocked) {