sgi/build-test-*: adapt the help messages to the new product names

Adapt the help messages in the test build scripts to the new product
names. In addition to this, use helper functions to simplify the
script code.

Change-Id: I4680c8b0864a4c79a00ea0a380a4d5bb88bc05b5
Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>