doc: Split the User Guide into multiple files

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>

1 files changed, 136 insertions, 0 deletions

diff --git a/docs/getting_started/prerequisites.rst b/docs/getting_started/prerequisites.rst
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+Prerequisites
+=============
+
+This document describes the software requirements for building |TF-A| for
+AArch32 and AArch64 target platforms.
+
+It may possible to build |TF-A| with combinations of software packages that are
+different from those listed below, however only the software described in this
+document can be officially supported.
+
+Build Host
+----------
+
+|TF-A| can be built using either a Linux or a Windows machine as the build host.
+
+A relatively recent Linux distribution is recommended for building |TF-A|. We
+have performed tests using Ubuntu 16.04 LTS (64-bit) but other distributions
+should also work fine as a base, provided that the necessary tools and libraries
+can be installed.
+
+.. _prerequisites_toolchain:
+
+Toolchain
+---------
+
+|TF-A| can be built with any of the following *cross-compiler* toolchains that
+target the Armv7-A or Armv8-A architectures:
+
+- GCC >= 8.3-2019.03 (from the `Arm Developer website`_)
+- Clang >= 4.0
+- Arm Compiler >= 6.0
+
+In addition, a native compiler is required to build the supporting tools.
+
+.. note::
+   The software has also been built on Windows 7 Enterprise SP1, using CMD.EXE,
+   Cygwin, and Msys (MinGW) shells, using version 5.3.1 of the GNU toolchain.
+
+.. note::
+   For instructions on how to select the cross compiler refer to
+   :ref:`Performing an Initial Build`.
+
+.. _prerequisites_software_and_libraries:
+
+Software and Libraries
+----------------------
+
+The following tools are required to obtain and build |TF-A|:
+
+- An appropriate toolchain (see :ref:`prerequisites_toolchain`)
+- GNU Make
+- Git
+
+The following libraries must be available to build one or more components or
+supporting tools:
+
+- OpenSSL >= 1.0.1
+
+   Required to build the cert_create tool.
+
+The following libraries are required for Trusted Board Boot support:
+
+- mbed TLS == 2.16.2 (tag: ``mbedtls-2.16.2``)
+
+These tools are optional:
+
+- Device Tree Compiler (DTC) >= 1.4.6
+
+   Needed if you want to rebuild the provided Flattened Device Tree (FDT)
+   source files (``.dts`` files). DTC is available for Linux through the package
+   repositories of most distributions.
+
+- Arm `Development Studio 5 (DS-5)`_
+
+   The standard software package used for debugging software on Arm development
+   platforms and |FVP| models.
+
+Package Installation (Linux)
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+If you are using the recommended Ubuntu distribution then you can install the
+required packages with the following command:
+
+.. code:: shell
+
+    sudo apt install build-essential git libssl-dev
+
+The optional packages can be installed using:
+
+.. code:: shell
+
+    sudo apt install device-tree-compiler
+
+Supporting Files
+----------------
+
+TF-A has been tested with pre-built binaries and file systems from `Linaro
+Release 19.06`_. Alternatively, you can build the binaries from source using
+instructions in :ref:`Performing an Initial Build`.
+
+.. _prerequisites_get_source:
+
+Getting the TF-A Source
+-----------------------
+
+Source code for |TF-A| is maintained in a Git repository hosted on
+TrustedFirmware.org. To clone this repository from the server, run the following
+in your shell:
+
+.. code:: shell
+
+    git clone "https://review.trustedfirmware.org/TF-A/trusted-firmware-a" && (cd "trusted-firmware-a" && mkdir -p .git/hooks && curl -Lo `git rev-parse --git-dir`/hooks/commit-msg https://review.trustedfirmware.org/tools/hooks/commit-msg; chmod +x `git rev-parse --git-dir`/hooks/commit-msg)
+
+This will clone the Git repository also install a *commit hook* that
+automatically inserts appropriate *Change-Id:* lines at the end of your
+commit messages. These change IDs are required when committing changes that you
+intend to push for review via our Gerrit system.
+
+You can read more about Git hooks in the *githooks* page of the Git documentation,
+available at: https://git-scm.com/docs/githooks
+
+Alternatively, you can clone without the commit hook using:
+
+.. code:: shell
+
+    git clone "https://review.trustedfirmware.org/TF-A/trusted-firmware-a"
+
+--------------
+
+*Copyright (c) 2019, Arm Limited. All rights reserved.*
+
+.. _Arm Developer website: https://developer.arm.com/open-source/gnu-toolchain/gnu-a/downloads
+.. _Linaro Release Notes: https://community.arm.com/dev-platforms/w/docs/226/old-release-notes
+.. _Linaro instructions: https://community.arm.com/dev-platforms/w/docs/304/arm-reference-platforms-deliverables
+.. _Development Studio 5 (DS-5): https://developer.arm.com/products/software-development-tools/ds-5-development-studio
+.. _Linaro Release 19.06: http://releases.linaro.org/members/arm/platforms/19.06