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/iproute2/Makefile

@@ -19,7 +19,7 @@ PKG_BUILD_DEPENDS:=iptables
 PKG_LICENSE:=GPL-2.0
 PKG_CPE_ID:=cpe:/a:iproute2_project:iproute2
 
-PKG_BUILD_FLAGS:=gc-sections
+PKG_BUILD_FLAGS:=gc-sections lto
 
 include $(INCLUDE_DIR)/kernel.mk
 include $(INCLUDE_DIR)/package.mk
@@ -173,7 +173,6 @@ define Build/Configure
 		> $(PKG_BUILD_DIR)/include/SNAPSHOT.h
 endef
 
-TARGET_CFLAGS += -flto
 TARGET_LDFLAGS += -Wl,--as-needed
 TARGET_CPPFLAGS += -I$(STAGING_DIR)/usr/include/libnl-tiny