Back in 2012,
/proc/last_kmsg (1) used to be a dump of kernel console messages, exported after reboots by Android's
ram_console functionality (CONFIG_ANDROID_RAM_CONSOLE), which used
persistent_ram (CONFIG_ANDROID_PERSISTENT_RAM) to handle a block of RAM that wasn't erased across reboots (2).
persistent_ram was based on Linux upstream RAMOOPS code which logged panic and oops messages to RAM. RAMOOPS was deprecated in favor of PSTORE (3) (memory handling) and PSTORE_RAM (4) (logging panics/oops to
pstore/dmesg-ramoops). Later PSTORE_CONSOLE support was added (5, 6) to log not only the panics/oops, but all kernel console messages to
pstore/console-ramoops-N which can be read later by mounting
pstore (7). So the Android specific RAM_CONSOLE (8, 9) and PERSISTENT_RAM (10) were utterly removed.
pstore also offers logging userspace messages (PSTORE_PMSG) (11) and other features (12). Plus it supports multiple backends for persistent storage if supported by hardware, though SoC's on Android devices only support RAM storage (
ramoops driver, AFAIK).
So it's not possible to enable
last_kmsg on an upstream kernel, instead
pstore offers a more versatile functionality.
mount: 'pstore'->'/mnt/pstore': No such device
It shows that your kernel isn't built with
pstore support. All you need to do is use a kernel built with
CONFIG_PSTORE_CONSOLE and optionally
CONFIG_PSTORE_RAM (though buggy). Android's
init by-default mounts
/sys/fs/pstore (13) from where
dmesg-ramoops (on panics/oops only) and
console-ramoops can be read.
You can search forums for a custom ROM shipped with such kernel. Or if you have kernel source, you can build yourself on a Linux machine.
FURTHER READING: Reading kernel logs