unbrick armStoneA8 after wrong nand write command.

First of all, pressing 's' is not as simple as the standard "any key" when U-Boot starts normally. The 's' must be pressed during a rather short time period at startup. Therefore we recommend setting the keyboard repeat rate to maximum and then press and hold 's' and while 's' is repeatedly sent to the board, switch the power on. You may even have to try it several times. But if there is never any reaction, then you have hit the jackpot and yes, then NBoot is gone. Then you have to send in the board to F&S after getting an RMA. The linked page will tell you whether the board can be sent in directly or must be sent via your distributor.

I'm still trying to figure out how this could happen. If you insert the zero at this place, then it is used as the size, i.e. as the number of bytes to write, and the $(filesize) is ignored. But a size of zero is caught, it should not go through to writing any data (where the 0 might be wrongly interpreted as 4GB). In addition we have a software write protection in place that should reject any writing to the low sectors of the NAND flash. To modify NBoot in the current U-Boot version, you even have to write to a completely different NAND device, as the flash is virtually split into two devices. So I'm completely puzzled how this was possible.