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_guardian_defconfig b/configs/am335x_guardian_defconfig
index 6723574..2b28cce 100644
--- a/configs/am335x_guardian_defconfig
+++ b/configs/am335x_guardian_defconfig
@@ -28,6 +28,8 @@
 CONFIG_SPL_ETH_SUPPORT=y
 CONFIG_SPL_I2C_SUPPORT=y
 CONFIG_SPL_MUSB_NEW_SUPPORT=y
+CONFIG_SPL_NAND_DRIVERS=y
+CONFIG_SPL_NAND_ECC=y
 CONFIG_SPL_NET_SUPPORT=y
 CONFIG_SPL_NET_VCI_STRING="Guardian U-Boot SPL"
 CONFIG_SPL_POWER_SUPPORT=y