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/drivers/block/Makefile b/drivers/block/Makefile
index a551591..98a34c0 100644
--- a/drivers/block/Makefile
+++ b/drivers/block/Makefile
@@ -5,7 +5,7 @@
 
 obj-$(CONFIG_$(SPL_TPL_)BLK) += blk-uclass.o
 
-ifndef CONFIG_$(SPL_)BLK
+ifndef CONFIG_$(XPL_)BLK
 obj-$(CONFIG_SPL_LEGACY_BLOCK) += blk_legacy.o
 endif