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/axm_defconfig b/configs/axm_defconfig
index b4bdc79..123ae40 100644
--- a/configs/axm_defconfig
+++ b/configs/axm_defconfig
@@ -8,6 +8,7 @@
 CONFIG_SYS_EXTRA_OPTIONS="AT91SAM9G20,MACH_TYPE=2068,BOARD_AXM"
 CONFIG_BOOTDELAY=3
 CONFIG_SPL=y
+CONFIG_SPL_NAND_SUPPORT=y
 CONFIG_HUSH_PARSER=y
 # CONFIG_CMD_BDI is not set
 CONFIG_CMD_BOOTZ=y