build-scp.sh: add support for product group

SCP now allows organizing a class of products under a product
group. Update the build-scp.sh script to accomodate for this
change. In addition to this, update the config files of the
reference design platforms to specify the product group.

Signed-off-by: Raghuram Thammiraju <raghuram.thammiraju@arm.com>
Change-Id: Ie44aa4a8dd09e67cf41adf80e2d3da68b5a6f8b1