[PATCH] Select NAND embedded environment from board configuration

The current NAND Bootloader setup forces the environment
variables to be in line with the bootloader. This change
enables the configuration to be made in the board include
file instead so that it can be individually enabled.

Signed-off-by: Nick Spence <nick.spence@freescale.com>
Signed-off-by: Stefan Roese <sr@denx.de>