spl: Makefile: Define SPL_ earlier

This Makefile variable can be used in the architecture's main Makefile but
at present it is not set up until later. Set it just before this Makefile is
included.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

scripts/Makefile.spl

@@ -35,6 +35,12 @@ else
 SPL_BIN := u-boot-spl
 endif
 
+ifdef CONFIG_SPL_BUILD
+SPL_ := SPL_
+else
+SPL_ :=
+endif
+
 include $(srctree)/config.mk
 include $(srctree)/arch/$(ARCH)/Makefile