autotest/factory_PreFinalCheck: add verified boot key matching test (from ToT to factory branch)

autotest/factory_PreFinalCheck: add verified boot key matching test (from ToT to factory branch)

This is cherry-picked from: http://codereview.chromium.org/6677018

Using dev_debug_vboot to verify all the keys are correct.
NOTE: verifying needs to read from system flashrom, and that takes
10-20 seconds on Alex for each trial (pressing space).

Before this CL, PreFinalCheck takes 1-2 seconds for each run;
by applying this CL, each run needs 10+ seconds.

BUG=chrome-os-partner:2634
TEST=on alex with OS in partition B signed with ssd key -> pass
     OS in B signed with recovery key -> fail

Committed: http://chrome-svn/viewvc/chromeos?view=rev&revision=38e4b9e

[Nick Sanders, 2011-03-25 05:31:51]

lgtm