refactor(sgi): replace all references to "sgi" within scripts

Currently, some of the platform utility scripts use "sgi" as a prefix to
name certain variables. As these platforms have moved away from "sgi",
update the prefix to "nrd" for the same.

Signed-off-by: Rohit Mathew <Rohit.Mathew@arm.com>
Change-Id: I0fa094e6d6fb08972b3c7d246b3b9341b50fa2ab
4 files changed