sd-image: Verifies the FAT partition before copying it.

This is to ensure `mtools`-based operations don't wreck the FS.

nixos/modules/installer/cd-dvd/sd-image.nix

@@ -135,6 +135,8 @@ in
 
         # Copy the populated /boot into the SD image
         (cd boot; mcopy -bpsvm -i ../bootpart.img ./* ::)
+        # Verify the FAT partition before copying it.
+        fsck.vfat -vn bootpart.img
         dd conv=notrunc if=bootpart.img of=$img seek=$START count=$SECTORS
       '';
     }) {};