I had the same issue for a long time finally managed to partially solve it using a simple UBoot script. I have flashed the nv-UBoot so I have full controll of the UBoot console, check
https://wiki.archlinux.org/index.php/Samsung_Chromebook_(ARM)#How_to_flash_U-Boot.
For me the /boot partition is the first on the eMMC (mmcblk0p1, mmc 0:1), and the root partition is the second on the eMMC (mmcblk0p2).
Create a file boot.scr containing:
$this->bbcode_second_pass_code('', '
setenv 1 'setenv bootargs root=/dev/mmcblk0p2 rootfstype=ext4 rootwait rw quiet; mmc dev 0; ext2load mmc 0:1 42000000 vmlinux.uimg; bootm 42000000'
if sleep 8; then vboot_test poweroff; fi
')
Then install the uboot-tools package and run as root:
$this->bbcode_second_pass_code('', 'mkimage -A arm -T script -C none -n 'Chromebook Boot Script' -d {path to boot.scr file} /boot/boot.scr.uimg')
This will create the script file that UBoot must run upon startup.
In the UBoot Prompt.Autoload the script.
$this->bbcode_second_pass_code('', '
setenv arch_boot 'mmc dev 0; ext2load mmc 0:1 40000000 boot.scr.uimg; source 40000000'
setenv bootcmd 'run arch_boot'
saveenv
')
Now your Chromebook will startup and hang on UBoot for 8 seconds, after that it will poweroff, its a hack. When you actually want to boot your Chromebook you have to press
Ctrl+C, and then you will need to run the enviroment variable command named "1".
$this->bbcode_second_pass_code('', 'run 1')
There is still an issue which has never happened to me but is possible in theory. If a random key is pressed and then the laptop powers on, the UBoot console hangs and the script wont run. Im still workng on this but it is a really rare case.