[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/storage/eeprom_raw.cpp

@@ -2,7 +2,7 @@
  * Copyright (C) OpenTX
  *
  * Based on code named
- *   th9x - http://code.google.com/p/th9x 
+ *   th9x - http://code.google.com/p/th9x
  *   er9x - http://code.google.com/p/er9x
  *   gruvin9x - http://code.google.com/p/gruvin9x
  *
@@ -63,16 +63,12 @@ uint8_t eepromWriteBuffer[EEPROM_BUFFER_SIZE] __DMA;
 
 void eepromWaitReadStatus()
 {
-  while (eepromReadStatus() == 0) {
-    SIMU_SLEEP(5/*ms*/);
-  }
+  while (eepromReadStatus() == 0 && SIMU_SLEEP(5/*ms*/)) { }
 }
 
 void eepromWaitTransferComplete()
 {
-  while (!eepromIsTransferComplete()) {
-    SIMU_SLEEP(5/*ms*/);
-  }
+  while (!eepromIsTransferComplete() && SIMU_SLEEP(5/*ms*/)) { }
 }
 
 void eepromEraseBlock(uint32_t address, bool blocking=true)