build_kernel_image.sh: Fix shellcheck warnings

Let shellcheck now that the variable can contain multiple files.

BUG=b:283830492
TEST=Pass pre-commit test

Change-Id: Idb39f1d9c819e9f4e951010a41e030239bb3c1b8
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/4553596
Commit-Queue: Ricardo Ribalda <ribalda@chromium.org>
Tested-by: Ricardo Ribalda <ribalda@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Cindy Lin <xcl@google.com>
1 file changed