logrotate: only install logrotate.conf if not present in target_skeleton

Allow platforms to provide a custom logrotate.conf in their target_skeleton.

package/logrotate/logrotate.mk

@@ -18,7 +18,9 @@ $(LOGROTATE_DIR)/$(LOGROTATE_BINARY): $(LOGROTATE_DIR)/.source
 
 $(TARGET_DIR)/$(LOGROTATE_TARGET_BINARY): $(LOGROTATE_DIR)/$(LOGROTATE_BINARY)
 	$(MAKE) PREFIX=$(TARGET_DIR) -C $(LOGROTATE_DIR) install
-	$(INSTALL) -m 0644 package/logrotate/logrotate.conf $(TARGET_DIR)/etc/logrotate.conf
+	if [ ! -f $(TARGET_DIR)/etc/logrotate.conf ]; then
+		$(INSTALL) -m 0644 package/logrotate/logrotate.conf $(TARGET_DIR)/etc/logrotate.conf;
+	fi
 	$(INSTALL) -d -m 0755 $(TARGET_DIR)/etc/logrotate.d
 
 logrotate: popt $(TARGET_DIR)/$(LOGROTATE_TARGET_BINARY)