firmware/2lib: Add error message for ctrl-U when usb boot is not enabled

Currently, we only have a beep when the user presses ctrl-u.  Adding
an error message to clarify what the error is.

BRANCH=puff, zork
TEST=boot into developer mode
     make sure "boot from external disk" doesn't appear
     press ctrl+U and make sure dialog box with error appears

Cq-Depend: chromium:2359765
Signed-off-by: Shelley Chen <>
Change-Id: Id8fad79802cb61883758e8c7561163cc17fb0f87
Tested-by: Shelley Chen <>
Reviewed-by: Yu-Ping Wu <>
Reviewed-by: Joel Kitching <>
Commit-Queue: Shelley Chen <>
2 files changed