sign_official_build: Parallelize multi-model LOEM firmware signing

Parallelize the LOEM firmware signing loop in
resign_firmware_shellball() to sign models concurrently across worker
processes (NUM_JOBS, defaulting to nproc).

Key changes:
- Validate that all output_name and bios_image entries are non-empty.
- Validate that all output_name entries in signer_config.csv are distinct.
- Validate that the bios_image-to-ec_image mapping is unique.
- Concurrently execute resign_firmware_image() in parallel across workers
  using spawn_worker() and wait_pids().
- Ensure parallel safety by operating on an isolated temp bios copy and
  atomically replacing the shared image.

BUG=b:532539923
TEST=make runtests
BRANCH=none
TAG=agy
CONV=705e5b3f-acf0-4ebd-91be-9a3d57b9198a

Change-Id: I81695c2f21eb736b9c947acc4436cc8eab65eb27
Signed-off-by: Yu-Ping Wu <yupingso@google.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/8276432
Tested-by: Yu-Ping Wu <yupingso@chromium.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Commit-Queue: Yu-Ping Wu <yupingso@chromium.org>