global: Rename SPL_ to XPL_

Use XPL_ as the symbol to indicate an SPL build. This means that SPL_ is
no-longer set.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/fs/fat/Makefile b/fs/fat/Makefile
index f84efac..7414b36 100644
--- a/fs/fat/Makefile
+++ b/fs/fat/Makefile
@@ -1,5 +1,5 @@
 # SPDX-License-Identifier: GPL-2.0+
 #
 
-obj-$(CONFIG_$(SPL_)FS_FAT) = fat.o
-obj-$(CONFIG_$(SPL_)FAT_WRITE) = fat_write.o
+obj-$(CONFIG_$(XPL_)FS_FAT) = fat.o
+obj-$(CONFIG_$(XPL_)FAT_WRITE) = fat_write.o