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/devkit8000_defconfig b/configs/devkit8000_defconfig
index 4eae268..593e9ca 100644
--- a/configs/devkit8000_defconfig
+++ b/configs/devkit8000_defconfig
@@ -12,6 +12,7 @@
 CONFIG_SPL_NAND_DRIVERS=y
 CONFIG_SPL_NAND_ECC=y
 CONFIG_SPL_NAND_SIMPLE=y
+CONFIG_SPL_NAND_BASE=y
 CONFIG_SPL_OS_BOOT=y
 # CONFIG_CMD_IMI is not set
 CONFIG_CMD_SPL=y