Convert CONFIG_SPL_NOR_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/x600_defconfig b/configs/x600_defconfig
index a9604ad..b953524 100644
--- a/configs/x600_defconfig
+++ b/configs/x600_defconfig
@@ -4,6 +4,7 @@
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
 CONFIG_BOOTDELAY=3
 CONFIG_SPL=y
+CONFIG_SPL_NOR_SUPPORT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="X600> "
 CONFIG_AUTOBOOT_KEYED=y