Recovery kernel by modifying kernel directly

Two test are involved: firmware_CorruptBothKernelAB and
firmware_RollbackKernel. Improve recovery step from using command
chromeos-install to directly modifying kernel. This may save many
times.

BUG=chrome-os-partner:13323
CQ-DEPEND=31692
TEST=.run_remote_tests.sh --remote="172.30.210.85" --board=link --servo
    firmware_RollbackKernel/control$ (or firmware_CorruptBothKernelAB)

Change-Id: I66819a0ed77d20f313f1407ddf14229f29085a6e
Reviewed-on: https://gerrit.chromium.org/gerrit/31693
Reviewed-by: Tom Wai-Hong Tam <waihong@chromium.org>
Commit-Ready: Tom Wai-Hong Tam <waihong@chromium.org>
Tested-by: Tom Wai-Hong Tam <waihong@chromium.org>
3 files changed