vboot: use recovery button as dev mode switch confirmation

We don't allow ENTER from a USB keyboard as the confirmation
in the switch from normal to developer mode.

For devices that have a physical recovery button, we require
a recovery button press instead.  For other devices, we
require that ENTER be pressed on the internal keyboard.

This prevents an "evil keyboard" attack in which a USB keyboard
(or other USB device pretending to be a keyboard) sends a
control-D/ENTER sequence shortly after every boot (followed
by more evil keys).  In that situation, when users power-on in
recovery mode, they will be forced to dev mode even if it
was not their intention.  Further attacks are easy at
that point.

TESTING.  On a panther device:

1. powered on with recovery button pressed -> booted in recovery mode
2. pressed control-D on external USB keyboard -> got to ToDev? screen
3. pressed ENTER -> system beeped
4. pressed recovery button -> system rebooted in DEV mode

... all as expected


1. powered on with recovery button pressed and HELD recovery button
2. pressed control-D -> system beeped

TEST=manual (see commit message)

Change-Id: Ib986d00d4567c2d447f8bbff0e5ccfec94596aa7
Reviewed-on: https://chromium-review.googlesource.com/182241
Reviewed-by: Luigi Semenzato <semenzato@chromium.org>
Tested-by: Luigi Semenzato <semenzato@chromium.org>
Commit-Queue: Luigi Semenzato <semenzato@chromium.org>
7 files changed