Restore multicore build support with kernel-bootstrap

This was removed as part of the simplify refactor, severely slowing
down qemu and bare-metal builds. Restoring it brings us back to the
same build times that we saw before the refactor.

steps/improve/finalize_job_count.sh

@@ -0,0 +1,14 @@
+#!/bin/sh
+#
+# SPDX-FileCopyrightText: 2024 Gábor Stefanik <netrolller.3d@gmail.com>
+#
+# SPDX-License-Identifier: GPL-3.0-or-later
+#
+# Finalize job count once SMP support is available
+
+cat >> /steps/bootstrap.cfg <<- EOF
+JOBS=${FINAL_JOBS}
+EOF
+
+. /steps/bootstrap.cfg
+. /steps/env