configs/acmesystems_arietta_g25: update kernel and at91bootstrap3

- configs/acmesystems_arietta_g25_128mb_defconfig,
  configs/acmesystems_arietta_g25_256mb_defconfig :
  update default configuration files for both 128MB and 256MB version

- board/acmesystems/arietta-g25/genimage.cfg
  board/acmesystems/arietta-g25/post-image.sh :
  improve 'post-image.sh' and 'genimage.cfg' used to build sdcard.img

- board/acmesystems/arietta-g25/readme.txt :
  update documentation for Arietta G25

Signed-off-by: Biagio Montaruli <biagio.hkr@gmail.com>
Acked-by: Angelo Compagnucci <angelo.compagnucci@gmail.com>
Reviewed-by: Angelo Compagnucci <angelo.compagnucci@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>