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/am3517_evm_defconfig b/configs/am3517_evm_defconfig
index 7a798f7..f235978 100644
--- a/configs/am3517_evm_defconfig
+++ b/configs/am3517_evm_defconfig
@@ -25,6 +25,7 @@
 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_SPL_POWER_SUPPORT is not set
 CONFIG_SYS_PROMPT="AM3517_EVM # "