swconfig: Activate LTO compile option

This decreases the size of the swconfig application by 25% on MIPS BE.

old:
16,916 /sbin/swconfig

new:
12,565 /sbin/swconfig

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
(cherry picked from commit e926681387)

package/network/config/swconfig/Makefile

@@ -23,6 +23,9 @@ define Package/swconfig
   TITLE:=Switch configuration utility
 endef
 
+TARGET_CFLAGS += -flto
+TARGET_LDFLAGS += -flto=jobserver
+
 TARGET_CPPFLAGS := \
 	-D_GNU_SOURCE \
 	-I$(STAGING_DIR)/usr/include/libnl-tiny \