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This patch reworks the section about booting an EL3 payload in the
User Guide:

 - Centralize all EL3 payload related information in the same
   section.

 - Mention the possibility to program the EL3 payload in flash memory
   and execute it in place.

 - Provide model parameters for both the Base and Foundation FVPs.

 - Provide some guidance to boot an EL3 payload on Juno.

Change-Id: I975c8de6b9b54ff4de01a1154cba63271d709912