Add crossystem support for nvram_cleared and kern_nv

Fix try_b processing

And move key block flags check up in LoadFirmware(), which speeds up
boot when the dev switch is off because it doesn't do a signature
check and then throw it out.

BUG=12282
TEST=build firmware, try by hand

Review URL: http://codereview.chromium.org/6596081

Change-Id: I10474e9e0ae324906dfe02a351347d04ce847f67
4 files changed