MAGIC value for powerReason should be stored in RTC backup on Horus (#7096)

- Allows the radio main application to start correctly when coming from an app in RAM
- Refactoring of the `UNEXPECTED_SHUTDOWN()` process
- Saves the RTC battery as BACKUP RAM is not needed on normal shutdown
- On PWR OFF on an ALERT, the radio won't show the next alert before shutting down
- Bootloader flashable through the SD Manager

radio/src/targets/taranis/board.cpp

@@ -36,7 +36,7 @@ void watchdogInit(unsigned int duration)
   IWDG->KR = 0x5555;      // Unlock registers
   IWDG->PR = 3;           // Divide by 32 => 1kHz clock
   IWDG->KR = 0x5555;      // Unlock registers
-  IWDG->RLR = duration;       // 1.5 seconds nominal
+  IWDG->RLR = duration;
   IWDG->KR = 0xAAAA;      // reload
   IWDG->KR = 0xCCCC;      // start
 }
@@ -222,7 +222,7 @@ void boardOff()
 
 #if defined(PWR_BUTTON_PRESS)
   while (pwrPressed()) {
-    wdt_reset();
+    WDG_RESET();
   }
 #endif
 
@@ -234,7 +234,7 @@ void boardOff()
   __disable_irq();
 
   while (1) {
-    wdt_reset();
+    WDG_RESET();
 #if defined(PWR_BUTTON_PRESS)
     // X9E/X7 needs watchdog reset because CPU is still running while
     // the power key is held pressed by the user.