Fix STM32 RTC Alarm interrupts.  They were being enabled BEFORE the interrupt system was being initialized.

Back out last RTC alarm changes.  I am mistaken, the interrupts are enabled by stm32[l4]_exti_alarm().

serial.c:  Fix a race condition noted by Stefan Kolb.  Between the test if the TX buffer is full and entering a critical section, bytes may be removed from the TX buffer making the wait unnecessary.  The unnecessary wait is an inefficiency, but not really a problem.  But with USB CDC/ACM it can be a problem because the entire TX buffer may be emptied when we lose the race.  If that happens that uart_putxmitchar() can hang waiting for data to be removed from an empty TX buffer.