Update (2014-11-20): After rebooting the phone, the the sshd (openssh-server) failed to start again (some error message like “chroot /var/run/sshd” failed). So the reliable way is still using dropbear.
Update (2014-11-06): I can install openssh-server successfully under gunroot. No special tricks (don’t why it failed in my first trial) except changing the port to > 1024 (/etc/ssh/sshd_config) and chmod og-rwx /etc/ssh/*. After the modification, issue /etc/init.d/ssh start