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/am335x_baltos_defconfig b/configs/am335x_baltos_defconfig
index f41f6bb..40d9e00 100644
--- a/configs/am335x_baltos_defconfig
+++ b/configs/am335x_baltos_defconfig
@@ -21,6 +21,8 @@
 CONFIG_SPL_FS_EXT4=y
 CONFIG_SPL_I2C_SUPPORT=y
 CONFIG_SPL_MTD_SUPPORT=y
+CONFIG_SPL_NAND_DRIVERS=y
+CONFIG_SPL_NAND_ECC=y
 CONFIG_SPL_POWER_SUPPORT=y
 CONFIG_SPL_WATCHDOG_SUPPORT=y
 CONFIG_SPL_YMODEM_SUPPORT=y