mxs: mxsboot: Move sdcard BCB header to 4 sectors offset

The MX23 Boot ROM does blindly load from 2048 offset while the MX28
does parse the BCB header to known where to load the image from. We
move the BCB header to 4 sectors offset so same code can be used by
both SoCs avoiding code duplication.

This idea was given by Marek Vasut <marex@denx.de>

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>