configs/imx28evk: Generate the standard sdcard.img

Generate the standard sdcard.img format that can be directly copied
to the SD card.

Remove the custom create-boot-sd.sh script and update the readme.txt
file accordingly.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
[Thomas: further tweaks to readme.txt.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>