mx28evk: Add 'nandboot' environment command

This reads the kernel, ftd and boot into ubifs filesystem. While on
that, the SD firmware filename definition has been moved next to the
other SD related commands.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Reviewed-by: Fabio Estevam <fabio.estevam@freescale.com>