scripts: Rename Makefile.spl to Makefile.xpl

Rename this file to indicate that it refers to any non-U-Boot-proper
phase, not just SPL, which is the phase immediately before U-Boot
proper.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/Makefile b/Makefile
index 300cee2..4378993 100644
--- a/Makefile
+++ b/Makefile
@@ -2094,7 +2094,7 @@
 	@:
 
 spl/u-boot-spl: tools prepare $(if $(CONFIG_SPL_OF_CONTROL),dts/dt.dtb)
-	$(Q)$(MAKE) obj=spl -f $(srctree)/scripts/Makefile.spl all
+	$(Q)$(MAKE) obj=spl -f $(srctree)/scripts/Makefile.xpl all
 
 spl/sunxi-spl.bin: spl/u-boot-spl
 	@:
@@ -2113,14 +2113,14 @@
 	$(TPL_SIZE_CHECK)
 
 tpl/u-boot-tpl: tools prepare $(if $(CONFIG_TPL_OF_CONTROL),dts/dt.dtb)
-	$(Q)$(MAKE) obj=tpl -f $(srctree)/scripts/Makefile.spl all
+	$(Q)$(MAKE) obj=tpl -f $(srctree)/scripts/Makefile.xpl all
 
 vpl/u-boot-vpl.bin: vpl/u-boot-vpl
 	@:
 	$(VPL_SIZE_CHECK)
 
 vpl/u-boot-vpl: tools prepare $(if $(CONFIG_TPL_OF_CONTROL),dts/dt.dtb)
-	$(Q)$(MAKE) obj=vpl -f $(srctree)/scripts/Makefile.spl all
+	$(Q)$(MAKE) obj=vpl -f $(srctree)/scripts/Makefile.xpl all
 
 TAG_SUBDIRS := $(patsubst %,$(srctree)/%,$(u-boot-dirs) include)