uboot-imx

MX51: vision2: Fix build for vision2 board.

config.mk should not be used in board directory and should be removed.
Use the same approach for building the image as other MX51/MX53 boards.

After this change vision2 board can be built again.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>