Check if kbd_mode even exists before resetting the modes.
Run dpkg --configure -a in the preparation script.
Always auto-unmount with -y when exiting the chroot.
croutonversion: Print URL on browser launch error.
Specify a second -u to upgrade the chroot release (dangerous).
Fix the keyboard mode after upgrades as well.
Enable signed boot verification if usb/legacy boot are disabled.

Otherwise, suggest to run a command but don't actually do it.
All of these boot options are opt-in, so if the others are set, the user
probably doesn't want crouton to be setting dev_boot_signed_only behind their
back.