sign_official_build: support signing 'base' image type

BRANCH=signer
BUG=chromium:512940
TEST=Tested with the following command:

$ ./sign_official_build.sh base chromiumos_base_image.bin ../../tests/devkeys \
    chromiumos_base_image_signed.bin ../../tests/devkeys/key.versions

Change-Id: Ife2284a6ca82f4306ca26278159859928c0ff2b5
Reviewed-on: https://chromium-review.googlesource.com/293636
Reviewed-by: Bill Richardson <wfrichar@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: Amey Deshpande <ameyd@google.com>
Commit-Queue: Amey Deshpande <ameyd@google.com>
1 file changed