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/dwc3/Makefile b/drivers/usb/dwc3/Makefile
index a46b682..a085c9d 100644
--- a/drivers/usb/dwc3/Makefile
+++ b/drivers/usb/dwc3/Makefile
@@ -6,11 +6,11 @@
 
 obj-$(CONFIG_USB_DWC3_GADGET)		+= gadget.o ep0.o
 
-obj-$(CONFIG_$(SPL_)USB_DWC3_AM62)	+= dwc3-am62.o
+obj-$(CONFIG_$(XPL_)USB_DWC3_AM62)	+= dwc3-am62.o
 obj-$(CONFIG_USB_DWC3_OMAP)		+= dwc3-omap.o
 obj-$(CONFIG_USB_DWC3_MESON_G12A)	+= dwc3-meson-g12a.o
 obj-$(CONFIG_USB_DWC3_MESON_GXL)	+= dwc3-meson-gxl.o
-obj-$(CONFIG_$(SPL_)USB_DWC3_GENERIC)		+= dwc3-generic.o
+obj-$(CONFIG_$(XPL_)USB_DWC3_GENERIC)		+= dwc3-generic.o
 obj-$(CONFIG_USB_DWC3_UNIPHIER)		+= dwc3-uniphier.o
 obj-$(CONFIG_USB_DWC3_LAYERSCAPE)	+= dwc3-layerscape.o
 obj-$(CONFIG_USB_DWC3_PHY_OMAP)		+= ti_usb_phy.o