build-arm-tf.sh: add new variable to define platform variant name

Platforms can have multiple variants while building for TF-A. For
example, rdn2 platform (which is the base variant) has two more variants
- rdn2cfg1 and rdn2cfg2. When building TF-A for a platform variant, the
TBBR enabled flag is assigned from platform's target config variable
instead of platform variant's target config variable. To correctly pick
the platform variant's target config variable, add a new environment
variable `ARM_TF_BUILD_VARIANT` in arm-tf build script to account for
the platform variants. If ARM_TF_BUILD_VARIANT is defined, then set TBBR
enabled flag from the platform variant's target variable instead of
platform's target variable.

Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>
Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>
Change-Id: Ia519f7e1620352026a969d7b2492821656f79fa8