configs/acmesystems_aria_g25: update and add genimage handling

configs/acmesystems_aria_g25: update and add genimage handling

- configs/acmesystems_aria_g25_128mb_defconfig,
  configs/acmesystems_aria_g25_256mb_defconfig :
  update default configuration files for both 128MB and 256MB version

- board/acmesystems/aria-g25/genimage.cfg
  board/acmesystems/aria-g25/post-image.sh :
  add support for genimage in order to build sdcard.img

- board/acmesystems/aria-g25/readme.txt : update documentation
  for Aria G25 and add notes about how to build sdcard.img

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