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/utils/e2fsprogs/Makefile

@@ -23,7 +23,7 @@ PKG_BUILD_DEPENDS:=util-linux e2fsprogs/host
 PKG_INSTALL:=1
 
 PKG_BUILD_PARALLEL:=1
-PKG_BUILD_FLAGS:=gc-sections
+PKG_BUILD_FLAGS:=gc-sections lto
 
 include $(INCLUDE_DIR)/package.mk
 include $(INCLUDE_DIR)/host-build.mk
@@ -143,9 +143,7 @@ $(call Package/e2fsprogs)
   DEPENDS:= +e2fsprogs
 endef
 
-TARGET_CFLAGS += $(FPIC) -flto
-
-TARGET_LDFLAGS += -flto
+TARGET_CFLAGS += $(FPIC)
 
 CONFIGURE_ARGS += \
 	--disable-testio-debug \