Add support for hello_world TA

Signed-off-by: Victor Chong <victor.chong@linaro.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>
Reviewed-by: David Brown <david.brown@linaro.org>
Tested-by: Victor Chong <victor.chong@linaro.org> (qemu)
Tested-by: Joakim Bech <joakim.bech@linaro.org> (RPi3 with mods
to build_ta_helloworld_qemu.sh for a 64-bit build)