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/wb50n_defconfig b/configs/wb50n_defconfig
index 32d4496..b89bf11 100644
--- a/configs/wb50n_defconfig
+++ b/configs/wb50n_defconfig
@@ -18,6 +18,7 @@
 CONFIG_USE_BOOTARGS=y
 CONFIG_BOOTARGS="rw rootfstype=ubifs ubi.mtd=6 root=ubi0:rootfs"
 CONFIG_SPL_NAND_SUPPORT=y
+CONFIG_SPL_NAND_DRIVERS=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y
 CONFIG_CMD_MEMTEST=y