problem with resetting backup domain clears clocking options set up before in *rcc.c

use INITS flag to avoid magic reg value to detect power up reset state of rtc
correct a problem clearing interrupt flags (they weren't) which prevented an alarm from ever being used more than once per reset cycle

arch/arm/src/stm32l4/stm32l4_exti_alarm.c

@@ -86,17 +86,17 @@ static int stm32l4_exti_alarm_isr(int irq, void *context)
 {
   int ret = OK;
 
-  /* Clear the pending EXTI interrupt */
-
-  putreg32(EXTI1_RTC_ALARM, STM32L4_EXTI1_PR);
-
-  /* And dispatch the interrupt to the handler */
+  /* Dispatch the interrupt to the handler */
 
   if (stm32l4_exti_callback)
     {
       ret = stm32l4_exti_callback(irq, context);
     }
 
+  /* Clear the pending EXTI interrupt */
+
+  putreg32(EXTI1_RTC_ALARM, STM32L4_EXTI1_PR);
+
   return ret;
 }