build-scripts/build-tf-a-tests.sh: Support for building multiple

platform variants

A new parameter "TF_A_PLATFORM_VARIANT" was introduced in TF-A that
allows retaining the build of multiple platform variants. This change
reserves the same support in TFTF build script. This change should be
applied together with patch [1].

[1] https://gerrit.oss.arm.com/c/css/src/fw/tftf/+/306868



Signed-off-by: Jerry Wang <Jerry.Wang4@arm.com>
Change-Id: Iec564cf9c47562148a5d568687469080d68bcfaf