board/freescale: use the common genimage template

board/freescale: use the common genimage template

The boards imx6dlsabreauto, imx6dlsabresd, imx6qsabreauto, imx6qsabresd,
imx6sxsabresd were referring to board/freescale/create-boot-sd.sh for
the sdcard image generation.

This commit:
* Removes board/freescale/create-boot-sd.sh
* Uses the common Freescale genimage template instead.
  The offset of the first partition was 1MB in the script and is
  modified to 8MB in the template.
* Fixes the readme.txt file accordingly

Signed-off-by: Georges Savoundararadj <savoundg@gmail.com>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>