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/iw/Makefile

@@ -18,7 +18,7 @@ PKG_HASH:=f167bbe947dd53bb9ebc0c1dcef5db6ad73ac1d6084f2c6f9376c5c360cc4d4e
 PKG_MAINTAINER:=Felix Fietkau <nbd@nbd.name>
 PKG_LICENSE:=GPL-2.0
 
-PKG_BUILD_FLAGS:=gc-sections
+PKG_BUILD_FLAGS:=gc-sections lto
 
 include $(INCLUDE_DIR)/package.mk
 
@@ -48,8 +48,7 @@ TARGET_CPPFLAGS:= \
 	-I$(STAGING_DIR)/usr/include/libnl-tiny \
 	$(TARGET_CPPFLAGS) \
 	-DCONFIG_LIBNL20 \
-	-D_GNU_SOURCE \
-	-flto
+	-D_GNU_SOURCE
 
 ifeq ($(BUILD_VARIANT),full)
   TARGET_CPPFLAGS += -DIW_FULL
@@ -58,7 +57,7 @@ endif
 
 MAKE_FLAGS += \
 	CFLAGS="$(TARGET_CPPFLAGS) $(TARGET_CFLAGS)" \
-	LDFLAGS="$(TARGET_LDFLAGS) -flto" \
+	LDFLAGS="$(TARGET_LDFLAGS)" \
 	NL1FOUND="" NL2FOUND=Y \
 	NLLIBNAME="libnl-tiny" \
 	LIBS="-lm -lnl-tiny" \