buildroot

configs/warp7: Use common script for generating boot image

Use the common board/freescale/common/post-image.sh script instead of
a custom one.

As the final image binary is called "sdcard.img" update the documentation
accordingly.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>