sign_nv_cbootimage: Update script to match script from master branch

CQ-DEPEND=CL:325370, CL:325382
BUG=chrome-os-partner:43018
BRANCH=None
TEST=Verified that signed image boots on dev-fused system.

Change-Id: Ice6cfce11fbcababd896a17e9546bcbc75b4a970
Signed-off-by: Furquan Shaikh <furquan@google.com>
Reviewed-on: https://chromium-review.googlesource.com/325381
Trybot-Ready: Furquan Shaikh <furquan@chromium.org>
Tested-by: Furquan Shaikh <furquan@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Furquan Shaikh <furquan@chromium.org>
3 files changed