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/sama5d3xek_nandflash_defconfig b/configs/sama5d3xek_nandflash_defconfig
index 9b70231..eba49c6 100644
--- a/configs/sama5d3xek_nandflash_defconfig
+++ b/configs/sama5d3xek_nandflash_defconfig
@@ -29,6 +29,7 @@
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_SPL_NAND_SUPPORT=y
+CONFIG_SPL_NAND_DRIVERS=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMI is not set