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/chiliboard_defconfig b/configs/chiliboard_defconfig
index 15d8487..1b08e75 100644
--- a/configs/chiliboard_defconfig
+++ b/configs/chiliboard_defconfig
@@ -22,6 +22,8 @@
 CONFIG_DEFAULT_FDT_FILE="am335x-chiliboard.dtb"
 CONFIG_ARCH_MISC_INIT=y
 CONFIG_SPL_I2C_SUPPORT=y
+CONFIG_SPL_NAND_DRIVERS=y
+CONFIG_SPL_NAND_ECC=y
 CONFIG_SPL_POWER_SUPPORT=y
 CONFIG_SPL_WATCHDOG_SUPPORT=y
 CONFIG_CMD_ASKENV=y