configs/acmesystems_arietta_g25: bump Linux and AT91Bootstrap versions

* Bump Linux version to 4.19
* Use default 'genimage' post-image script (support/scripts/genimage.sh)
  to build sdcard.img
* Bump AT91bootstrap version to 3.8.13

Fixes:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/269139084
  https://gitlab.com/buildroot.org/buildroot/-/jobs/269139083



Signed-off-by: Biagio Montaruli <biagio.hkr@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>