Since RCPL 21 or 22, rebuilding busybox makes kernel to be rebuilt from checking out kernel sources.
1 Configure platform project.
${WRL_HOME}/wrlinux-5/wrlinux/configure \
         --enable-build=production \
         --enable-board=stm-spear13xx \
         --enable-kernel=preempt-rt \
         --enable-rootfs=glibc_small \
         --enable-ccache=yes \
         --with-rcpl-version=0022 \
         --with-sstate-dir=~/tmp/make \
         --enable-jobs=2 \
         --enable-parallel-pkgbuilds=2
2 make fs
3 Add a fs_final*.sh script.
 touch layers/local/conf/image_final/fs_final_test.sh
 chmod a+x layers/local/conf/image_final/fs_final_test.sh
4 Rebuid busybox.
  make -C build busybox.rebuild
5 make fs
  You will see kernel build from checking out kernel source.