Changed SD library for nrf52 and got S&F working (#6382)

- Changed library from deprecated arduino SD library to wrapper of SdFat library
- Fixed some bugs in the S&F code for SD card storage
Now the S&F functionality works on nrf52 (tested on rak4631 with RAK15002)

variants/rak4631/variant.h

@@ -138,6 +138,12 @@ static const uint8_t SCK = PIN_SPI_SCK;
 #define SPI_MISO PIN_SPI1_MISO
 #define SDCARD_CS (26)
 
+// Some settings for the SdFat library to optimize flash usage
+#define SDFAT_FILE_TYPE 1 // only support FAT16/FAT32, not exFAT
+#define CHECK_FLASH_PROGRAMMING                                                                                                  \
+    0 // this reduces flash usage but may cause higher power usage when sd card is idle TODO:Check if power usage is higher
+#define MAINTAIN_FREE_CLUSTER_COUNT 1 // maintain free cluster count
+
 /*
  * eink display pins
  */