Create bootstub signing utility.
Create bootstub signing utility.

This tool uses cbootimage to sign a bootstub firmware image.
This makes calling cbootimage from a number of scripts easier.

BUG=chromium-os:11981
TEST=run cros_sign_bootstub and sign a recovery image and write it
to a board using cros_write_firmware which hasn't been submitted yet.

Change-Id: I4b3a9c763d0659d0244eb4d6ed2f0a38154d00c4

This CL depends on: http://codereview.chromium.org/6591056/

Review URL: http://codereview.chromium.org/6592053

mgireesh05/dev-util,coreos/dev-util,coreos/dev-util,mgireesh05/dev-util,mgireesh05/dev-util,coreos/dev-util,marineam/coreos-dev-util,marineam/coreos-dev-util