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/am43xx_evm_rtconly_defconfig b/configs/am43xx_evm_rtconly_defconfig
index 54085fa..1c40d51 100644
--- a/configs/am43xx_evm_rtconly_defconfig
+++ b/configs/am43xx_evm_rtconly_defconfig
@@ -16,6 +16,8 @@
 # CONFIG_MISC_INIT_R is not set
 CONFIG_VERSION_VARIABLE=y
 CONFIG_SPL_MTD_SUPPORT=y
+CONFIG_SPL_NAND_DRIVERS=y
+CONFIG_SPL_NAND_ECC=y
 CONFIG_SPL_OS_BOOT=y
 CONFIG_CMD_SPL=y
 CONFIG_CMD_SPL_NAND_OFS=0x00100000