Delete an SD card file.
The path starts in the root directory of the SD card.
If Shred Level is more than 0, the contents of the file are overwritten with random bytes the specified number of times before it is deleted.
The intention is to make it (much) harder to recover the contents than if the file was simply deleted.
Security note: shredding will only provide basic protection on 'journalling' filesystems, but most external storage uses FAT32 at the time of writing.