Add support for dev_boot_usb flag

BUG=chromium-os:17433
TEST=make && make runtests.  Additional manual tests:

0. Insert a valid dev-signed USB key.

1. Boot with dev switch off.
`crossystem dev_boot_usb` should print 0.

2. Flip dev switch on.
`crossystem dev_boot_usb` should print 0.
Ctrl+U at dev screen should beep, but not boot USB.

3. Type `crossystem dev_boot_usb=1`.  Should succeed.
`crossystem dev_boot_usb` should print 1.

4. Reboot system.
At the dev mode warning, press Ctrl+U
System should boot from USB key
`crossystem dev_boot_usb` should print 0.

5. Flip dev switch off.
`crossystem dev_boot_usb` should print 0.

6. Flip dev switch on.
`crossystem dev_boot_usb` should print 0.

Note that this does not apply to Cr-48, Alex, or ZGB.

Change-Id: Idf85fdd642f38f531c89e5fa5b1679e84936d4da
Reviewed-on: http://gerrit.chromium.org/gerrit/3875
Reviewed-by: Bill Richardson <wfrichar@chromium.org>
Reviewed-by: Stefan Reinauer <reinauer@google.com>
Tested-by: Randall Spangler <rspangler@chromium.org>
6 files changed