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/am3517_crane_defconfig b/configs/am3517_crane_defconfig
index 8ad2c84..4155df5 100644
--- a/configs/am3517_crane_defconfig
+++ b/configs/am3517_crane_defconfig
@@ -10,6 +10,9 @@
 CONFIG_SPL=y
 CONFIG_BOOTDELAY=10
 # CONFIG_SPL_FS_EXT4 is not set
+CONFIG_SPL_NAND_DRIVERS=y
+CONFIG_SPL_NAND_ECC=y
+CONFIG_SPL_NAND_SIMPLE=y
 CONFIG_HUSH_PARSER=y
 # CONFIG_CMDLINE_EDITING is not set
 CONFIG_SYS_PROMPT="AM3517_CRANE # "
@@ -30,7 +33,6 @@
 CONFIG_MTD=y
 CONFIG_MTD_RAW_NAND=y
 CONFIG_SYS_NAND_BUSWIDTH_16BIT=y
-CONFIG_SPL_NAND_SIMPLE=y
 CONFIG_CONS_INDEX=3
 CONFIG_SYS_NS16550=y
 CONFIG_USB=y