Move the STM32 rtc drivers from core to library

To avoid further dependency issue with the core.

Signed-off-by: Frederic Pillon <frederic.pillon@st.com>

Author: fpistm

cores/arduino/board.h

@@ -13,7 +13,6 @@
 #include "dwt.h"
 #include "hw_config.h"
 #include "low_power.h"
-#include "rtc.h"
 #include "stm32_eeprom.h"
 #include "timer.h"
 #include "uart.h"