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/busybox/Makefile

@@ -16,6 +16,7 @@ PKG_HASH:=542750c8af7cb2630e201780b4f99f3dcceeb06f505b479ec68241c1e6af61a5
 
 PKG_BUILD_DEPENDS:=BUSYBOX_CONFIG_PAM:libpam
 PKG_BUILD_PARALLEL:=1
+PKG_BUILD_FLAGS:=lto
 PKG_CHECK_FORMAT_SECURITY:=0
 
 PKG_LICENSE:=GPL-2.0
@@ -103,9 +104,6 @@ ifeq ($(BUILD_VARIANT),selinux)
   LDLIBS += selinux sepol
 endif
 
-TARGET_CFLAGS += -flto
-TARGET_LDFLAGS += -flto=jobserver -fuse-linker-plugin
-
 MAKE_VARS :=
 MAKE_FLAGS += \
 	EXTRA_CFLAGS="$(TARGET_CFLAGS) $(TARGET_CPPFLAGS)" \