Convert CONFIG_SPL_NAND_DRIVERS et al to Kconfig

This converts the following to Kconfig:
   CONFIG_SPL_NAND_DRIVERS
   CONFIG_SPL_NAND_ECC
   CONFIG_SPL_NAND_SIMPLE

Signed-off-by: Adam Ford <aford173@gmail.com>
diff --git a/configs/am3517_evm_defconfig b/configs/am3517_evm_defconfig
index bd76642..7a798f7 100644
--- a/configs/am3517_evm_defconfig
+++ b/configs/am3517_evm_defconfig
@@ -22,6 +22,9 @@
 # CONFIG_SPL_FS_EXT4 is not set
 # CONFIG_SPL_I2C_SUPPORT is not set
 CONFIG_SPL_MTD_SUPPORT=y
+CONFIG_SPL_NAND_DRIVERS=y
+CONFIG_SPL_NAND_ECC=y
+CONFIG_SPL_NAND_SIMPLE=y
 CONFIG_SPL_OS_BOOT=y
 # CONFIG_SPL_POWER_SUPPORT is not set
 CONFIG_SYS_PROMPT="AM3517_EVM # "
@@ -56,7 +59,6 @@
 CONFIG_SYS_NAND_BUSWIDTH_16BIT=y
 CONFIG_SYS_NAND_U_BOOT_LOCATIONS=y
 CONFIG_SYS_NAND_U_BOOT_OFFS=0x80000
-CONFIG_SPL_NAND_SIMPLE=y
 CONFIG_DM_ETH=y
 CONFIG_MII=y
 CONFIG_DRIVER_TI_EMAC=y