Convert CONFIG_SPL_NAND_SUPPORT to Kconfig

Move this option to Kconfig and tidy up existing uses.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/P2020RDB-PC_36BIT_NAND_defconfig b/configs/P2020RDB-PC_36BIT_NAND_defconfig
index a551bdf..1c3a12a 100644
--- a/configs/P2020RDB-PC_36BIT_NAND_defconfig
+++ b/configs/P2020RDB-PC_36BIT_NAND_defconfig
@@ -9,12 +9,14 @@
 CONFIG_SYS_EXTRA_OPTIONS="P2020RDB,NAND"
 CONFIG_BOOTDELAY=10
 CONFIG_SPL=y
+CONFIG_SPL_NAND_SUPPORT=y
 CONFIG_TPL=y
 CONFIG_TPL_ENV_SUPPORT=y
 CONFIG_TPL_I2C_SUPPORT=y
 CONFIG_TPL_LIBCOMMON_SUPPORT=y
 CONFIG_TPL_LIBGENERIC_SUPPORT=y
 CONFIG_TPL_MPC8XXX_INIT_DDR_SUPPORT=y
+CONFIG_TPL_NAND_SUPPORT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_MMC=y
 CONFIG_CMD_SF=y