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/usb/host/Makefile b/drivers/usb/host/Makefile
index 8dad36f..07deb4f 100644
--- a/drivers/usb/host/Makefile
+++ b/drivers/usb/host/Makefile
@@ -3,7 +3,7 @@
 # (C) Copyright 2000-2007
 # Wolfgang Denk, DENX Software Engineering, wd@denx.de.
 
-ifdef CONFIG_$(SPL_)DM_USB
+ifdef CONFIG_$(XPL_)DM_USB
 obj-y += usb-uclass.o
 obj-$(CONFIG_SANDBOX) += usb-sandbox.o
 endif