[STM32F4][DISCO-F429ZI] LSE not soldered. Use LSI RTC

Author: Erwan GOURIOU

hal/targets/hal/TARGET_STM/TARGET_STM32F4/TARGET_DISCO_F429ZI/device.h

@@ -48,7 +48,8 @@
 #define DEVICE_SPISLAVE         1
 
 #define DEVICE_RTC              1
-#define DEVICE_RTC_LSI          0
+#define DEVICE_RTC_LSI          1
+/* LSE not soldered by default : RTC date backup not guaranteed */
 
 #define DEVICE_PWMOUT           1