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/system/mtd/Makefile

@@ -18,6 +18,7 @@ PKG_LICENSE:=GPL-2.0+
 PKG_LICENSE_FILES:=
 
 PKG_FLAGS:=nonshared
+PKG_BUILD_FLAGS:=lto
 
 include $(INCLUDE_DIR)/package.mk
 
@@ -36,8 +37,7 @@ endef
 target=$(firstword $(subst -, ,$(BOARD)))
 
 MAKE_FLAGS += TARGET="$(target)"
-TARGET_CFLAGS += -Dtarget_$(target)=1 -Wall -flto
-TARGET_LDFLAGS += -flto=jobserver
+TARGET_CFLAGS += -Dtarget_$(target)=1 -Wall
 
 ifdef CONFIG_MTD_REDBOOT_PARTS
   MAKE_FLAGS += FIS_SUPPORT=1