signer: Handle zstd squashfs compression

BUG=b:193618692
TEST=sign_official_build.sh can handle zstd-compressed Android image

BRANCH=none
Signed-off-by: Satoshi Niwa <niwa@google.com>
Cq-Depend: chrome-internal:4024687
Change-Id: Ie01e93e49da9b32245055f7e4b6fa4fb3fbefd8e
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/3066801
Tested-by: Satoshi Niwa <niwa@chromium.org>
Reviewed-by: Kazuhiro Inaba <kinaba@chromium.org>
Reviewed-by: Yury Khmel <khmel@chromium.org>
Auto-Submit: Satoshi Niwa <niwa@chromium.org>
Commit-Queue: Satoshi Niwa <niwa@chromium.org>
diff --git a/scripts/image_signing/sign_android_image.sh b/scripts/image_signing/sign_android_image.sh
index e90f51e..a7d0fc6 100755
--- a/scripts/image_signing/sign_android_image.sh
+++ b/scripts/image_signing/sign_android_image.sh
@@ -314,6 +314,8 @@
     compression_flags="-comp gzip"
   elif [[ "${compression}" == "Compression lz4" ]]; then
     compression_flags="-comp lz4 -Xhc -b 256K"
+  elif [[ "${compression}" == "Compression zstd" ]]; then
+    compression_flags="-comp zstd"
   else
     die "Unexpected compression type: ${compression}"
   fi