Switch N1SDP bootflow to trusted board boot

This patch adds changes to build tf-a with trusted board boot
enabled in build-arm-tf.sh file and package the output files in
build-firmware-image.sh file.

Signed-off-by: sahil <sahil@arm.com>
Change-Id: I4244ab428786c8cdb477829b97dfa9df8d9f2d10