You can use the already available ramoops. For that, you have to manually edit the kernel config (make kernel_menuconfig). Activate the pstore filesystem (Misc Filesystems) and enable the option Log panic/oops to a RAM buffer.
Afterwards, change the default cmdline string in boot options for the kernel to mem=384M ramoops.mem_address=0x5fba4000 ramoops.mem_size=0x45bfff
After experiencing a crash-based reboot, mount the pstore pseudo-fs mount -t pstore pstore /mnt/. The files located /mnt contain your crash log.
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