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/taurus_defconfig b/configs/taurus_defconfig
index fa0fb41..f5fd14c 100644
--- a/configs/taurus_defconfig
+++ b/configs/taurus_defconfig
@@ -41,6 +41,7 @@
 CONFIG_SPL_DM_SPI_FLASH=y
 CONFIG_SPL_NAND_DRIVERS=y
 CONFIG_SPL_NAND_ECC=y
+CONFIG_SPL_NAND_BASE=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="U-Boot> "
 CONFIG_SYS_XTRACE="n"