I've successful built and tested a working U-Boot version for Zyxel NSA310S/320S. Please try and report!
It should be loaded with UART in serial console:
Warning: this image is not for flashing to NAND.
It should be loaded with UART in serial console:
kwboot -t -B 115200 /dev/ttyUSB0 -b uboot.2014.07-tld-4.nsa310s.uart.kwb
Warning: this image is not for flashing to NAND.