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/P1020RDB-PC_36BIT_NAND_defconfig b/configs/P1020RDB-PC_36BIT_NAND_defconfig
index 82afb26..95c3fc9 100644
--- a/configs/P1020RDB-PC_36BIT_NAND_defconfig
+++ b/configs/P1020RDB-PC_36BIT_NAND_defconfig
@@ -9,6 +9,7 @@
 CONFIG_SYS_EXTRA_OPTIONS="P1020RDB_PC,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