Ok I ran into this issue on my chromebook. You need to go into the bios and select the kernel in /boot as the boot location.
What I did longterm to make my C730 chromebook usable was get a 60gb sd card. Cut half of it off (because most is empty plastic and then stick it in the sd card slot. This means it doesnt stick out the sides. Then I use the 60gb sd card as the root drive and the 15gb internal as swap.