Convert CONFIG_SPL_NAND_DRIVERS et al to Kconfig

This converts the following to Kconfig:
   CONFIG_SPL_NAND_DRIVERS
   CONFIG_SPL_NAND_ECC
   CONFIG_SPL_NAND_SIMPLE

Signed-off-by: Adam Ford <aford173@gmail.com>
diff --git a/configs/wb45n_defconfig b/configs/wb45n_defconfig
index b07996c..398b5c3 100644
--- a/configs/wb45n_defconfig
+++ b/configs/wb45n_defconfig
@@ -19,6 +19,7 @@
 CONFIG_BOOTARGS="console=ttyS0,115200 earlyprintk rw noinitrd mem=64M rootfstype=ubifs root=ubi0:rootfs ubi.mtd=6"
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_SPL_NAND_SUPPORT=y
+CONFIG_SPL_NAND_DRIVERS=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y
 CONFIG_CMD_MEMTEST=y