I don't think the uboot can use network (tftp) on nsa310 as it has an external network chip and bodhi (he made the uboot also OpenWrt uses) never fixed that. Openwrt can use the network chip so this is a problem only when installing the first time and you are using uboot.
I didn't know the uboot in OpenWrt has no ide command. The only option for you is installing bodhi's uboot then. Use his uboot and follow the tutorial on the nsa310b page in the wiki.