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/cm_t43_defconfig b/configs/cm_t43_defconfig
index 1679b74..0d9afce 100644
--- a/configs/cm_t43_defconfig
+++ b/configs/cm_t43_defconfig
@@ -35,6 +35,7 @@
 CONFIG_SPL_DM_SPI_FLASH=y
 CONFIG_SPL_NAND_DRIVERS=y
 CONFIG_SPL_NAND_ECC=y
+CONFIG_SPL_NAND_BASE=y
 CONFIG_SPL_POWER_SUPPORT=y
 CONFIG_SPL_SPI_LOAD=y
 CONFIG_SYS_PROMPT="CM-T43 # "