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/gadget/udc/Makefile b/drivers/usb/gadget/udc/Makefile
index 467c566..4b6a8fd 100644
--- a/drivers/usb/gadget/udc/Makefile
+++ b/drivers/usb/gadget/udc/Makefile
@@ -2,9 +2,9 @@
 #
 # USB peripheral controller drivers
 
-ifndef CONFIG_$(SPL_)DM_USB_GADGET
+ifndef CONFIG_$(XPL_)DM_USB_GADGET
 obj-$(CONFIG_USB_DWC3_GADGET)	+= udc-core.o
 endif
 
-obj-$(CONFIG_$(SPL_)DM_USB_GADGET)	+= udc-core.o
+obj-$(CONFIG_$(XPL_)DM_USB_GADGET)	+= udc-core.o
 obj-y += udc-uclass.o