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/am335x_igep003x_defconfig b/configs/am335x_igep003x_defconfig
index 8bf23a6..f624bb4 100644
--- a/configs/am335x_igep003x_defconfig
+++ b/configs/am335x_igep003x_defconfig
@@ -25,6 +25,7 @@
 CONFIG_SPL_MTD_SUPPORT=y
 CONFIG_SPL_NAND_DRIVERS=y
 CONFIG_SPL_NAND_ECC=y
+CONFIG_SPL_NAND_BASE=y
 CONFIG_SPL_UBI=y
 CONFIG_SPL_UBI_MAX_VOL_LEBS=256
 CONFIG_SPL_UBI_MAX_PEB_SIZE=262144