Only advance kernel version when the booted partition has a valid key block

Change-Id: I879c18cc139f8a02bcaa6eb80437eaf836a5e851

BUG=chrome-os-partner:2597
TEST=manual

install pvt-signed image
flip dev switch on and boot
install dev-signed firmware
reboot
reboot
sudo bash
stop tcsd
tpmc read 0x1008 0x0d

bytes 6-10 should be 1 0 1 0, not FF FF FF FF

Review URL: http://codereview.chromium.org/6626020
13 files changed