bug discovered. temporary fix could be done in the several ways before flashing firmware:
1. add "insmod ipt_recent.o" in your /usr/local/sbin/post-boot script
2. set nvram recent_ftp_enable to 1 if you've enabled ftp server via web interface, commit nvram after
3. set both ssh_enable and nvram recent_ssh_enable to 1, commit nvram after. it will cause dropbear to start on boot time without any additional post-boot scripts
sorry for the inconvenience, folks