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/common/Makefile b/drivers/usb/common/Makefile
index 2e9353b..11cc465 100644
--- a/drivers/usb/common/Makefile
+++ b/drivers/usb/common/Makefile
@@ -3,7 +3,7 @@
 # (C) Copyright 2016 Freescale Semiconductor, Inc.
 #
 
-obj-$(CONFIG_$(SPL_)DM_USB) += common.o
+obj-$(CONFIG_$(XPL_)DM_USB) += common.o
 obj-$(CONFIG_USB_ISP1760) += usb_urb.o
 obj-$(CONFIG_USB_MUSB_HOST) += usb_urb.o
 obj-$(CONFIG_USB_MUSB_GADGET) += usb_urb.o