Thanks for the info. I just tested this on my Lantiq xrx200 o2 Box 6431 by forcing a kernel panic with
echo c > /proc/sysrq-trigger
and after the box came up again, /sys/kernel/debug/crashlog contained the last 16 kB of kernel messages including the crash message.
This should be documented on the wiki (and possibly merged upstream) because it is a really helpful tool. I wish I had known of this earlier. @nbd Thank you very much for writing this patch.
[edit]As @jow pointed out, @nbd wrote the patch. My apologies for the confusion.[/edit]