tf-a-tests.sh: do not use preset tf-a build options

There are preset TF-A build options used by the script that builss the
tf-a-tests test case. Remove those preset build options and allow TF-A
build options to be supplied from the config file.

Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>
Change-Id: Ibfc221cea6c34ac5da29f3db9378fd3b9fec71a1