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/config/firewall/Makefile

@@ -21,7 +21,7 @@ PKG_LICENSE:=ISC
 
 PKG_CONFIG_DEPENDS := CONFIG_IPV6
 
-PKG_BUILD_FLAGS:=gc-sections
+PKG_BUILD_FLAGS:=gc-sections lto
 
 include $(INCLUDE_DIR)/package.mk
 include $(INCLUDE_DIR)/cmake.mk
@@ -44,8 +44,6 @@ define Package/firewall/conffiles
 /etc/firewall.user
 endef
 
-TARGET_CFLAGS += -flto
-TARGET_LDFLAGS += -flto
 CMAKE_OPTIONS += $(if $(CONFIG_IPV6),,-DDISABLE_IPV6=1)
 
 define Package/firewall/install

package/network/config/netifd/Makefile

@@ -13,6 +13,8 @@ PKG_MAINTAINER:=Felix Fietkau <nbd@nbd.name>
 PKG_LICENSE:=GPL-2.0
 PKG_LICENSE_FILES:=
 
+PKG_BUILD_FLAGS:=lto
+
 include $(INCLUDE_DIR)/package.mk
 include $(INCLUDE_DIR)/cmake.mk
 
@@ -30,10 +32,7 @@ endef
 
 TARGET_CFLAGS += \
 	-I$(STAGING_DIR)/usr/include/libnl-tiny \
-	-I$(STAGING_DIR)/usr/include \
-	-flto
-
-TARGET_LDFLAGS += -flto -fuse-linker-plugin
+	-I$(STAGING_DIR)/usr/include
 
 CMAKE_OPTIONS += \
 	-DLIBNL_LIBS=-lnl-tiny \

package/network/config/swconfig/Makefile

@@ -13,6 +13,8 @@ PKG_RELEASE:=12
 PKG_MAINTAINER:=Felix Fietkau <nbd@nbd.name>
 PKG_LICENSE:=GPL-2.0
 
+PKG_BUILD_FLAGS:=lto
+
 include $(INCLUDE_DIR)/package.mk
 include $(INCLUDE_DIR)/kernel.mk
 
@@ -23,9 +25,6 @@ define Package/swconfig
   TITLE:=Switch configuration utility
 endef
 
-TARGET_CFLAGS += -flto
-TARGET_LDFLAGS += -flto=jobserver
-
 TARGET_CPPFLAGS := \
 	-D_GNU_SOURCE \
 	-I$(STAGING_DIR)/usr/include/libnl-tiny \