Code update BMC signed image keyword

Added:
    - Keyword to check if the BMC image is signed.

Currently the logic is kept simple and generic so that it can be
extended for host signing as well ahead.

Resolves  openbmc/openbmc-test-automation#1305

Change-Id: I97d9820bccacceb6361bf24d451ad22c15646522
Signed-off-by: George Keishing <gkeishin@in.ibm.com>
2 files changed