Add Juno instructions to user guide

This patch makes the Trusted Firmware build instructions in the
user guide platform independent.

FVP specific instructions have been grouped together under a new
section dedicated to FVP.

Juno specific instructions to build and run the Trusted Firmware,
UEFI and Linux have been added.

Change-Id: I9bfb1b9d732b1f73abbe29f68ac931e1773a4fd5