You can use uboot (trough serial console is, I think, possible to add temporally any boot parameter) or recompile Turris OS. None of which is good option. But kernel log is also logged by syslog. So you should be able to found whole kernel log in /var/log/messages. You can get it with some grep command, such as grep "kernel\[\]:" /var/log/messages.
For even better option, see syslog-ng documentation and configure it so that kernel log is logged also to separate file.