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Update user guide to use Linaro releases

Linaro produce monthly software releases for the Juno and AEMv8-FVP
platforms. These provide an integrated set of software components
that have been tested together on these platforms.

From now on, it is recommend that Trusted Firmware developers use the
Linaro releases (currently 15.06) as a baseline for the dependent
software components: normal world firmware, Linux kernel and device
tree, file system as well as any additional micro-controller firmware
required by the platform.

This patch updates the user guide to document this new process. It
changes the instructions to get the source code of the full software
stack (including Trusted Firmware) and updates the dependency build
instructions to make use of the build scripts that the Linaro releases
provide.

Change-Id: Ia8bd043f4b74f1e1b10ef0d12cc8a56ed3c92b6e