configs: Convert CONFIG_USE_NAND to CONFIG_NAND

The DA850-EVM and OMAPL138_LCDK both use checks for CONFIG_USE_NAND.
This patch changes these checks to CONFIG_NAND which is already defined
in Kconfig.  Since the OMAPL138_LCDK already had CONFIG_NAND defined in its
defconfig, it can be deleted from configs/omapl138_lcdk.h.

Signed-off-by: Adam Ford <aford173@gmail.com>