mx28evk: Introduce a new target for saving env vars to NAND
Introduce 'mx28evk_nand' target for saving environment variables into NAND.
The mx28evk board does not come with a NAND flash populated from the
factory. It comes with an empty slot (U23), which allows the insertion of a
48-pin TSOP flash device.
Tested with a K9LBG08U0D.
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com> Reviewed-by: Otavio Salvador <otavio@ossystems.com.br>