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This patch enables LTO for TF-A when compiled with GCC.
LTO is disabled by default and is enabled by
ENABLE_LTO=1 build option.

LTO is enabled only for aarch64 as there seem to be
a bug in the aarch32 compiler when LTO is enabled.

The changes in the makefiles include:
- Adding -flto and associated flags to enable LTO.
- Using gcc as a wrapper at link time instead of ld.
  This is recommended when using LTO as gcc internally
  takes care of invoking the necessary plugins for LTO.
- Adding switches to pass options to ld.
- Adding a flag to disable fix for erratum cortex-a53-843419
  unless explicitly enabled. This is needed because GCC
  seem to automatically add the erratum fix when used
  as a wrapper for LD.

Additionally, this patch updates the TF-A user guide with
the new build option.
Signed-off-by: zelalem-aweke <zelalem.aweke@arm.com>
Change-Id: I1188c11974da98434b7dc9344e058cd1eacf5468

The User Guide document has grown organically over time and
now covers a wide range of topics, making it difficult to
skim read and extract information from. Currently, it covers
these topics and maybe a couple more:

- Requirements (hardware, tools, libs)
- Checking out the repo
- Basic build instructions
- A comprehensive list of build flags
- FIP packaging
- Building specifically for Juno
- Firmware update images
- EL3 payloads
- Preloaded BL33 boot flow
- Running on FVPs
- Running on Juno

I have separated these out into a few groups that become new
documents. Broadly speaking, build instructions for the tools,
for TF-A generally, and for specific scenarios are separated.
Content relating to specific platforms (Juno and the FVPs are
Arm-specific platforms, essentially) has been moved into the
documentation that is specific to those platforms, under
docs/plat/arm.

Change-Id: Ica87c52d8cd4f577332be0b0738998ea3ba3bbec
Signed-off-by: Paul Beesley <paul.beesley@arm.com>