After using my Dualboot Acer Chromebook CB3-431 in ChromeOS, the ability to boot into Linux (GalliumOS) via the usual Ctrl-L command stopped functioning. Instead Ctrl-L resulted in beeps and I was effectively forced to re-enter ChromeOS.
The following fixed the problem for me and re-enabled my Chromebook to load Linux using Ctrl-L at start up.
1. Boot into ChromeOS using Ctrl-D and login
2. Open a terminal (Ctrl-alt-T) and re-enable boot from legacy and boot from USB
sudo crossystem dev_boot_usb=1 dev_boot_legacy=1
Now all is back to normal.