treewide: add support for "lto" in PKG_BUILD_FLAGS

This reduces open coding and allows to easily add a knob to enable
it treewide, where chosen packages can still opt-out via "no-lto".

Some packages used LTO, but not the linker plugin. This unifies 'em
all to attempt to produce better code.
Quoting man gcc(1):
"This improves the quality of optimization by exposing more code to the
link-time optimizer."

Also use -flto=auto instead of -flto=jobserver, as it's not guaranteed
that every buildsystem uses +$(MAKE) correctly.

Signed-off-by: Andre Heider <a.heider@gmail.com>

package/network/utils/nftables/Makefile

@@ -20,6 +20,8 @@ PKG_LICENSE_FILES:=COPYING
 PKG_FIXUP:=autoreconf
 PKG_INSTALL:=1
 
+PKG_BUILD_FLAGS:=lto
+
 include $(INCLUDE_DIR)/package.mk
 
 DISABLE_NLS:=
@@ -60,9 +62,6 @@ ifeq ($(BUILD_VARIANT),json)
   CONFIGURE_ARGS += --with-json
 endif
 
-TARGET_CFLAGS += -flto
-TARGET_LDFLAGS += -flto
-
 define Build/InstallDev
 	$(INSTALL_DIR) $(1)/usr/lib $(1)/usr/include
 	$(CP) $(PKG_INSTALL_DIR)/usr/lib/*.so* $(1)/usr/lib/