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>