LTO (Link-Time Optimisation) is an very useful feature which can
significantly reduce the size of U-Boot binaries. So far it has been
made available for selected ARM boards and sandbox.

However, incremental builds are much slower when LTO is used. For example,
an incremental build of sandbox takes 2.1 seconds on my machine, but 6.7
seconds with LTO enabled.

Add a LTO_BUILD=n parameter to the build, so it can be disabled during
development if needed, for faster builds.

Add some documentation about LTO while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>
Series-to: u-boot
Series-cc: trini, heinrich
Series-cc: Marek Behún <marek.behun@nic.cz>