Convert CONFIG_SPL_NAND_BASE et al to Kconfig

This converts the following to Kconfig:
   CONFIG_SPL_NAND_BASE
   CONFIG_SPL_NAND_IDENT

Signed-off-by: Adam Ford <aford173@gmail.com>
diff --git a/configs/wb50n_defconfig b/configs/wb50n_defconfig
index b89bf11..4bee60f 100644
--- a/configs/wb50n_defconfig
+++ b/configs/wb50n_defconfig
@@ -19,6 +19,7 @@
 CONFIG_BOOTARGS="rw rootfstype=ubifs ubi.mtd=6 root=ubi0:rootfs"
 CONFIG_SPL_NAND_SUPPORT=y
 CONFIG_SPL_NAND_DRIVERS=y
+CONFIG_SPL_NAND_BASE=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y
 CONFIG_CMD_MEMTEST=y