Set errno to ENOTEMPTY when removing a non-empty directory

The value that was previously set, EPERM, implies that the user didn't
have the (write) permission to remove the directory.

source/fatdir.c

@@ -136,7 +136,7 @@ int _FAT_unlink_r (struct _reent *r, const char *path) {
 			if (!_FAT_directory_isDot (&dirContents)) {
 				// The directory had something in it that isn't a reference to itself or it's parent
 				_FAT_unlock(&partition->lock);
-				r->_errno = EPERM;
+				r->_errno = ENOTEMPTY;
 				return -1;
 			}
 			nextEntry = _FAT_directory_getNextEntry (partition, &dirContents);