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/thuban_defconfig b/configs/thuban_defconfig
index 8402408..fa02206 100644
--- a/configs/thuban_defconfig
+++ b/configs/thuban_defconfig
@@ -32,6 +32,7 @@
 CONFIG_SPL_DM_SPI_FLASH=y
 CONFIG_SPL_NAND_DRIVERS=y
 CONFIG_SPL_NAND_ECC=y
+CONFIG_SPL_NAND_BASE=y
 CONFIG_SPL_SPI_LOAD=y
 CONFIG_SPL_WATCHDOG_SUPPORT=y
 CONFIG_SPL_YMODEM_SUPPORT=y