Config.in: add new option BR2_ENABLE_LTO to globally enable LTO

Some packages have a configure option to enable LTO. Add a global option
to enable/disable this in packages.

Note that we could instead (or in addition) pass -flto directly to the
compiler in the toolchain wrapper, but that has a *much* bigger impact,
so for the time being let's not do that.

No toolchain dependency is added for this, because all toolchains now
support LTO.
- LTO support was merged in GCC years before 4.6 (our minimal version)
  was released.
- Our oldest external toolchain, Sourcery ARM 2014.05, has LTO support.
- For our internal toolchains, it is supposedly regulated by
  BR2_GCC_ENABLE_LTO. However, that option only *enables* it - it
  doesn't disable LTO support when it's not provided. Since LTO support
  defaults to enabled for a long time already in GCC, in practice all
  Buildroot-generated toolchains do support LTO.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

Config.in

@@ -551,6 +551,25 @@ config BR2_OPTIMIZE_FAST
 
 endchoice
 
+config BR2_ENABLE_LTO
+	bool "build packages with link-time optimisation"
+	help
+	  Enable the link-time optimisation (LTO) option when building
+	  packages. Link-time optimisation re-runs optimisations at
+	  link time, which allows the compiler to do interprocedural
+	  analysis across compilation units and thus come with better
+	  results: smaller size and better performance.
+
+	  Note that this analysis is limited to statically linked
+	  object files and libraries.
+
+	  This option may significantly increase build times,
+	  sometimes 5 times longer, with only limited gains.
+
+	  At this time, this option only enables LTO in packages that
+	  have an explicit configuration option for it. Other packages
+	  always enable LTO, but most packages never enable LTO.
+
 config BR2_GOOGLE_BREAKPAD_ENABLE
 	bool "Enable google-breakpad support"
 	depends on BR2_INSTALL_LIBSTDCPP