configs/snps_archs38_vdk: use post-build script instead of overlay

configs/snps_archs38_vdk: use post-build script instead of overlay

In commit 2cdfa6c84951b25e805d329cb7bae833d333538f ("synopsys/axs10x:
Update /etc/inittab by post-build"), two Synopsys platforms were
changed to use a post-build script adding an extra getty in the
inittab instead of a rootfs overlay containing a custom
inittab. However, in this commit, configs/snps_archs38_vdk_defconfig
was not changed, even though it was using the same rootfs overlay.

This commit therefore adjusts configs/snps_archs38_vdk_defconfig to
also use the newly introduced post-build script, fixing the build of
configs/snps_archs38_vdk_defconfig.

Fixes:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/88314952

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>