Add Juno instructions to user guide
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

achingupta/arm-trusted-firmware,robixnai/arm-trusted-firmware,jcastillo-arm/arm-trusted-firmware,athoelke/arm-trusted-firmware,idl3r/arm-trusted-firmware,Leo-Yan/arm-trusted-firmware,vchong/arm-trusted-firmware,sandrine-bailleux/arm-trusted-firmware,jforissier/arm-trusted-firmware,96boards/arm-trusted-firmware,lsigithub/arm-trusted-firmware_public,ljerry/arm-trusted-firmware,idl3r/arm-trusted-firmware,hzhuang1/arm-trusted-firmware,ljerry/arm-trusted-firmware,athoelke/arm-trusted-firmware,sbranden/arm-trusted-firmware,vchong/arm-trusted-firmware,sbranden/arm-trusted-firmware,yatharth-arm/arm-trusted-firmware,achingupta/arm-trusted-firmware,davwan01/arm-trusted-firmware,lsigithub/arm-trusted-firmware_public,yatharth-arm/arm-trusted-firmware,mtk09422/arm-trusted-firmware,mtk09422/arm-trusted-firmware,jcastillo-arm/arm-trusted-firmware,robixnai/arm-trusted-firmware,sandrine-bailleux-arm/arm-trusted-firmware,96boards/arm-trusted-firmware,sandrine-bailleux-arm/arm-trusted-firmware,jforissier/arm-trusted-firmware,davwan01/arm-trusted-firmware,longsleep/arm-trusted-firmware-pine64,hzhuang1/arm-trusted-firmware,Leo-Yan/arm-trusted-firmware,longsleep/arm-trusted-firmware-pine64,sandrine-bailleux/arm-trusted-firmware