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/work_92105_defconfig b/configs/work_92105_defconfig
index 507aeb2..e884cf7 100644
--- a/configs/work_92105_defconfig
+++ b/configs/work_92105_defconfig
@@ -26,6 +26,7 @@
 CONFIG_SPL_BOARD_INIT=y
 CONFIG_SPL_SYS_MALLOC_SIMPLE=y
 CONFIG_SPL_NAND_SUPPORT=y
+CONFIG_SPL_NAND_DRIVERS=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_EEPROM=y
 # CONFIG_CMD_FLASH is not set