Makefile: savedefconfig: save in O= directory for out-of-tree builds

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

Makefile

@@ -603,7 +603,7 @@ defconfig: $(BUILD_DIR)/buildroot-config/conf outputmakefile
 
 savedefconfig: $(BUILD_DIR)/buildroot-config/conf outputmakefile
 	@mkdir -p $(BUILD_DIR)/buildroot-config
-	@$(COMMON_CONFIG_ENV) $< --savedefconfig=$(TOPDIR)/defconfig $(CONFIG_CONFIG_IN)
+	@$(COMMON_CONFIG_ENV) $< --savedefconfig=$(CONFIG_DIR)/defconfig $(CONFIG_CONFIG_IN)
 
 # check if download URLs are outdated
 source-check: allyesconfig