signer: continue to use --sb_extract for shellballs

crrev.com/c/3665477 started using `--unpack` rather than
`--sb_extract` for self-extracting firmware update bundles.
Unfortunately, existing prebuilt artifacts used by signer tests are
old and only support `--sb_extract`, and hence fail with recent
vboot_reference that includes this change.

This change simply reverts the option back to `--sb_extract` for
unpacking self-extracting bundles.

BRANCH=None
BUG=b:179170462
BUG=b:202397678
TEST=unittests

Change-Id: I528346a5d593caba7ca822820b9a38e994f0ea94
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/3792485
Commit-Queue: Josh Horwich <jhorwich@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Prameet Shah <phshah@chromium.org>
Tested-by: Josh Horwich <jhorwich@chromium.org>
Auto-Submit: Josh Horwich <jhorwich@chromium.org>

scripts/image_signing/common.sh

diff --git a/scripts/image_signing/common.sh b/scripts/image_signing/common.sh
index 85475d8..81bcac3 100644
--- a/scripts/image_signing/common.sh
+++ b/scripts/image_signing/common.sh
@@ -146,8 +146,8 @@
     return 1
   elif grep -q '^##CUTHERE##' "${input}"; then
     # Bundle supports self-extraction.
-    "${input}" --unpack "${output_dir}" ||
-      die "Extracting firmware autoupdate (--unpack) failed."
+    "${input}" --sb_extract "${output_dir}" ||
+      die "Extracting firmware autoupdate (--sb_extract) failed."
   else
     # Legacy bundle - try uudecode.
     uudecode -o - "${input}" | tar -C "${output_dir}" -zxf - 2>/dev/null ||