Updated dependencies for flash-image

Modifications in RSS, TFA, RSS-trusty and TFA-trusty **impact flash-image** so it needs to be rebuilt. When modifying TFA, RSS already needs to be rebuilt (and because RSS is rebuilt then flash-image needs to be as well), so there is no need to put `build-tfa.sh=build-flash-image.sh'.

Signed-off-by: Alex Dobrescu <alex.dobrescu@arm.com>