cros_update_firmware: Use parallel emerge

Use parallel emerge for firmware manifest build.

BUG=none
TEST=Test with zork f/w rebuild.

Change-Id: Iccad8bc10e5331ef8acaea39aab8a29707ad0e9e
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/2297021
Tested-by: Andrew McRae <amcrae@chromium.org>
Auto-Submit: Andrew McRae <amcrae@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
1 file changed