xpl: Define CONFIG_SPL_BUILD only for the SPL build

Make this define mean SPL only, not TPL, VPL, etc.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/scripts/Makefile.autoconf b/scripts/Makefile.autoconf
index d754402..c1eab2f 100644
--- a/scripts/Makefile.autoconf
+++ b/scripts/Makefile.autoconf
@@ -87,11 +87,11 @@
 
 tpl/u-boot.cfg: include/config.h FORCE
 	$(Q)mkdir -p $(dir $@)
-	$(call cmd,u_boot_cfg,-DCONFIG_XPL_BUILD -DCONFIG_SPL_BUILD -DCONFIG_TPL_BUILD)
+	$(call cmd,u_boot_cfg,-DCONFIG_XPL_BUILD -DCONFIG_TPL_BUILD)
 
 vpl/u-boot.cfg: include/config.h FORCE
 	$(Q)mkdir -p $(dir $@)
-	$(call cmd,u_boot_cfg,-DCONFIG_XPL_BUILD -DCONFIG_SPL_BUILD -DCONFIG_VPL_BUILD)
+	$(call cmd,u_boot_cfg,-DCONFIG_XPL_BUILD -DCONFIG_VPL_BUILD)
 
 include/autoconf.mk: u-boot.cfg
 	$(call cmd,autoconf)