stm32/fatfs_port: Fix bug when MICROPY_HW_ENABLE_RTC not enabled.

Prior to this patch, get_fattime() was calling a HAL RTC function with the
HW instance pointer as null because rtc_init_start() was never called.

Also marked it as a weak function, to allow a board to override it.

ports/stm32/fatfs_port.c

@@ -28,11 +28,16 @@
 #include "lib/oofatfs/ff.h"
 #include "rtc.h"
 
-DWORD get_fattime(void) {
+MP_WEAK DWORD get_fattime(void) {
+    #if MICROPY_HW_ENABLE_RTC
     rtc_init_finalise();
     RTC_TimeTypeDef time;
     RTC_DateTypeDef date;
     HAL_RTC_GetTime(&RTCHandle, &time, RTC_FORMAT_BIN);
     HAL_RTC_GetDate(&RTCHandle, &date, RTC_FORMAT_BIN);
     return ((2000 + date.Year - 1980) << 25) | ((date.Month) << 21) | ((date.Date) << 16) | ((time.Hours) << 11) | ((time.Minutes) << 5) | (time.Seconds / 2);
+    #else
+    // Jan 1st, 2018 at midnight. Not sure what timezone.
+    return ((2018 - 1980) << 25) | ((1) << 21) | ((1) << 16) | ((0) << 11) | ((0) << 5) | (0 / 2);
+    #endif
 }