Are you sure about that? I've got a Samsung chromebook (Snow) from about 3 years ago, and yet...

     localhost / # uname -a
     Linux localhost 3.8.11 #1 SMP Mon Sep 26 02:48:53 PDT 2016 armv7l ARMv7 Processor rev 4 (v7l)
     SAMSUNG EXYNOS5 (Flattened Device Tree) GNU/Linux

Recently built kernel. Unless things have changed from a few years back (when I last played with it), there are multiple (3) kernel partitions in ChromeOS:

https://www.chromium.org/chromium-os/chromiumos-design-docs/...

...and you can manually select between them by using the "cgpt" command from the shell:

So on an ARM device, you could say:

     sudo cgpt add -i 6 -P 5 -S 1 /dev/mmcblk0

If I'm not mistaken, this would mean to set partition 6 (a kernel) with a priority of 5 (compared to other partitions at boot), successful flag set to 1 on mmc block device 0-- (the internal mmc, where mmcblk1 is the external sdcard). I think if that kernel fails, that flips the successful flag to 0 and it falls back to the next priority kernel on the reboot. But like I said, it's been a while so don't quote me.

See more here: https://www.chromium.org/chromium-os/chromiumos-design-docs/...

Also, if you weren't aware, to get into the shell:

     ctrl-alt-T
     shell
     sudo bash
     cgpt --help

if you play with cgpt, be VERY careful. Also, if you're thinking of replacing kernels or whatnot, be aware that on most (all?) devices, the partitions are signed, so you'll have to turn off any checks first.