sgi/build-test-secureboot: align with changes to directory structure

The directory structure in pre-builts for storing the secure boot test
keys and images has changed. Align this script to those changes and
also to the changes in the naming of reference platforms.

Change-Id: Ieeeaec0fa6342bcb0f9a9263d6fff8f32115b112
Signed-off-by: Jagadeesh Ujja <jagadeesh.ujja@arm.com>