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>
diff --git a/scripts/image_signing/sign_official_build.sh b/scripts/image_signing/sign_official_build.sh
index 5272106..5bd23fe 100755
--- a/scripts/image_signing/sign_official_build.sh
+++ b/scripts/image_signing/sign_official_build.sh
@@ -26,6 +26,7 @@
 Usage: $PROG <type> input_image /path/to/keys/dir [output_image] [version_file]
 where <type> is one of:
              ssd  (sign an SSD image)
+             base (sign a base image, similar to an SSD image)
              recovery (sign a USB recovery image)
              factory (sign a factory install image)
              install (old alias to "factory")
@@ -718,7 +719,7 @@
 echo "Using kernel version: ${KERNEL_VERSION}"
 
 # Make all modifications on output copy.
-if [[ "${TYPE}" == "ssd" ]]; then
+if [[ "${TYPE}" == "ssd" || "${TYPE}" == "base" ]]; then
   sign_image_file "SSD" "${INPUT_IMAGE}" "${OUTPUT_IMAGE}" 2 \
     "${KEY_DIR}/kernel.keyblock" "${KEY_DIR}/kernel_data_key.vbprivk" \
     "${KEY_DIR}/kernel.keyblock" "${KEY_DIR}/kernel_data_key.vbprivk"