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/cdns3/Makefile b/drivers/usb/cdns3/Makefile
index 18d7190..d604785 100644
--- a/drivers/usb/cdns3/Makefile
+++ b/drivers/usb/cdns3/Makefile
@@ -4,8 +4,8 @@
 
 obj-$(CONFIG_USB_CDNS3)			+= cdns3.o
 
-cdns3-$(CONFIG_$(SPL_)USB_CDNS3_GADGET)	+= gadget.o ep0.o
+cdns3-$(CONFIG_$(XPL_)USB_CDNS3_GADGET)	+= gadget.o ep0.o
 
-cdns3-$(CONFIG_$(SPL_)USB_CDNS3_HOST)	+= host.o
+cdns3-$(CONFIG_$(XPL_)USB_CDNS3_HOST)	+= host.o
 
 obj-$(CONFIG_USB_CDNS3_TI)		+= cdns3-ti.o