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/dropbear/Makefile

@@ -23,7 +23,7 @@ PKG_CPE_ID:=cpe:/a:matt_johnston:dropbear_ssh_server
 
 PKG_BUILD_PARALLEL:=1
 PKG_ASLR_PIE_REGULAR:=1
-PKG_BUILD_FLAGS:=no-mips16 gc-sections
+PKG_BUILD_FLAGS:=no-mips16 gc-sections lto
 PKG_FIXUP:=autoreconf
 PKG_FLAGS:=nonshared
 
@@ -142,8 +142,7 @@ DB_OPT_CONFIG = \
 	DROPBEAR_SVR_AGENTFWD|CONFIG_DROPBEAR_AGENTFORWARD|1|0 \
 
 
-TARGET_CFLAGS += -DARGTYPE=3 -flto
-TARGET_LDFLAGS += -flto=jobserver
+TARGET_CFLAGS += -DARGTYPE=3
 
 db_opt_add     =echo '\#define $(1) $(2)' >> $(PKG_BUILD_DIR)/localoptions.h
 db_opt_replace =$(ESED) 's,^(\#define $(1)) .*$$$$,\1 $(2),g' $(PKG_BUILD_DIR)/sysoptions.h