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/services/lldpd/Makefile

@@ -21,6 +21,7 @@ PKG_CPE_ID:=cpe:/a:lldpd_project:lldpd
 
 PKG_FIXUP:=autoreconf
 PKG_BUILD_PARALLEL:=1
+PKG_BUILD_FLAGS:=lto
 PKG_INSTALL:=1
 
 include $(INCLUDE_DIR)/package.mk
@@ -111,7 +112,6 @@ CONFIGURE_ARGS += \
 	$(if $(CONFIG_LLDPD_WITH_SNMP),--with-snmp,) \
 	$(if $(CONFIG_USE_GLIBC),,--without-libbsd)
 
-TARGET_CFLAGS += -flto
-TARGET_LDFLAGS += -flto -Wl,--gc-sections,--as-needed
+TARGET_LDFLAGS += -Wl,--gc-sections,--as-needed
 
 $(eval $(call BuildPackage,lldpd))