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/taurus_defconfig b/configs/taurus_defconfig
index fa999a7..fa0fb41 100644
--- a/configs/taurus_defconfig
+++ b/configs/taurus_defconfig
@@ -39,6 +39,8 @@
 CONFIG_SPL_CRC32_SUPPORT=y
 CONFIG_SPL_NAND_SUPPORT=y
 CONFIG_SPL_DM_SPI_FLASH=y
+CONFIG_SPL_NAND_DRIVERS=y
+CONFIG_SPL_NAND_ECC=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="U-Boot> "
 CONFIG_SYS_XTRACE="n"