usign: Activate LTO compile option

This decreases the size of the usign application by 16% on MIPS BE.

old:
24,597 /usr/bin/usign

new:
20,501 /usr/bin/usign

Signed-off-by: Hauke Mehrtens <[email protected]>

package/system/usign/Makefile

@@ -30,6 +30,9 @@ define Package/usign
   TITLE:=OpenWrt signature verification utility
 endef
 
+TARGET_CFLAGS += -flto
+TARGET_LDFLAGS += -flto=jobserver
+
 CMAKE_OPTIONS += \
 	-DUSE_LIBUBOX=on