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.