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/hostapd/Makefile

@@ -32,7 +32,7 @@ PKG_CONFIG_DEPENDS:= \
 	CONFIG_DRIVER_11AX_SUPPORT \
 	CONFIG_WPA_ENABLE_WEP
 
-PKG_BUILD_FLAGS:=gc-sections
+PKG_BUILD_FLAGS:=gc-sections lto
 
 EAPOL_TEST_PROVIDERS:=eapol-test eapol-test-openssl eapol-test-wolfssl
 
@@ -587,8 +587,7 @@ TARGET_CPPFLAGS := \
 	-D_GNU_SOURCE \
 	$(if $(CONFIG_WPA_MSG_MIN_PRIORITY),-DCONFIG_MSG_MIN_PRIORITY=$(CONFIG_WPA_MSG_MIN_PRIORITY))
 
-TARGET_CFLAGS += -flto
-TARGET_LDFLAGS += -flto=jobserver -fuse-linker-plugin -lubox -lubus
+TARGET_LDFLAGS += -lubox -lubus
 
 ifdef CONFIG_PACKAGE_kmod-cfg80211
   TARGET_LDFLAGS += -lm -lnl-tiny