Introduce PLATFORM_VARIANT for juno
Created by: mdgzim01
The PLATFORM_VARIANT option has been enabled for Juno platform which will allow the user to choose to build for BOARD
or FVP
. BOARD is the default value.
If the PLATFORM_VARIANT is 'FVP', the bl1 and bl2 images will be built. If it is 'BOARD', the bl1-bypass and bl2 images will be built.