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/axm_defconfig b/configs/axm_defconfig
index 837e729..0a3ddf9 100644
--- a/configs/axm_defconfig
+++ b/configs/axm_defconfig
@@ -37,6 +37,7 @@
 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_CMD_BDI is not set