In order to boot a chromebook from USB you must turn on developer mode.
Note: this was tested with the following Chromebook.
Once in developer mode PRESS CTLR+SHIFT+T to open a CROSH terminal.
To enable booting signed images (CTLR+U) Run:
sudo crossystem dev_boot_usb=1
To enable booting other images (CTLR+L) Run:
sudo crossystem dev_boot_legacy=1
Once these settings are in place you can run any boot-able usb drive from the USB 2.0 slot.