UserAgent: Mozilla/5.0 (X11; CrOS x86_64 10830.0.2018) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/69.0.3476.0 Safari/537.36
Platform: 10830.0.2018_06_30_1245 (Developer Build - build) developer-build amd64-generic

Steps to reproduce the problem:
1. Build Chromium OS amd64-generic (with 4.14 kernel)
2. Install to flash drive
3. Try to boot from flash drive

What is the expected behavior?
Chromium OS boots

What went wrong?
Grub fails to load with a single "_" on screen.

Did this work before? No 

Chrome version: 69.0.3476.0  Channel: n/a
OS Version: 10830.0.2018
Flash Version: 

This is due to a bug in grub. A patch has already been merged upstream. I will submit a patch to Chromium OS to backport the fix.

https://git.savannah.gnu.org/cgit/grub.git/commit/?id=446794de8da4329ea532cbee4ca877bcafd0e534