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_sl50_defconfig b/configs/am335x_sl50_defconfig
index f15b5cc..6a9555a 100644
--- a/configs/am335x_sl50_defconfig
+++ b/configs/am335x_sl50_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_NET_SUPPORT=y
 CONFIG_SPL_NET_VCI_STRING="AM335x U-Boot SPL"
 CONFIG_SPL_OS_BOOT=y