drivers: Use CONFIG_XPL_BUILD instead of CONFIG_SPL_BUILD

Use the new symbol to refer to any 'SPL' build, including TPL and VPL

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/mtd/Makefile b/drivers/mtd/Makefile
index 10d575e..db01041 100644
--- a/drivers/mtd/Makefile
+++ b/drivers/mtd/Makefile
@@ -16,7 +16,7 @@
 mtd-$(CONFIG_HBMC_AM654) += hbmc-am654.o
 
 # U-Boot build
-ifeq ($(CONFIG_SPL_BUILD)$(CONFIG_TPL_BUILD),)
+ifeq ($(CONFIG_XPL_BUILD)$(CONFIG_TPL_BUILD),)
 
 ifneq ($(mtd-y),)
 obj-y += mtd.o