pkg/fatfs: use page-wise read() and write()
pkg/fatfs: use page-wise read() and write()

The FAT file system operates on sectors (pages), so use the new
mtd_write_page() and mtd_read_page() functions.

This also avoids the wrap-around after reading/writing past 4 GiB.

ant9000/RIOT,ant9000/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,authmillenon/RIOT,ant9000/RIOT,OTAkeys/RIOT,kYc0o/RIOT,kYc0o/RIOT,OTAkeys/RIOT,miri64/RIOT,kaspar030/RIOT,OTAkeys/RIOT,kYc0o/RIOT,authmillenon/RIOT,miri64/RIOT,kaspar030/RIOT,authmillenon/RIOT,jasonatran/RIOT,miri64/RIOT,OlegHahm/RIOT,ant9000/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,kaspar030/RIOT,jasonatran/RIOT,kaspar030/RIOT,miri64/RIOT,OlegHahm/RIOT,authmillenon/RIOT,OTAkeys/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,miri64/RIOT,ant9000/RIOT,kYc0o/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,kaspar030/RIOT,authmillenon/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,jasonatran/RIOT