refactor(sgi): rename variable name and path for prebuilts

Currently, reference design platforms such as RD-N1-Edge, RD-V1, and
SGI-575 are collectively known as "sgi platforms". SGI stands for System
Guidance Infrastructure and was initially associated with the SGI-575
platform. However, subsequent platforms released were under the Neoverse
reference design product name. To align with the Neoverse reference
design nomenclature, source files for these platforms for both Trusted
Firmware and Trusted Firmware Tests have moved away from SGI to Neoverse
RD/NRD/nrd in their naming convention.

As of now, the prebuilts for "sgi platforms" are stored within
$css_downloads/sgi/<platform_name> referenced by the variable
"sgi_prebuilts". In order to aid the move from "sgi", change the
prebuilt path to $tfa_downloads/neoverse_rd/<platform_name>.
Additionally, rename the prebuilt variable prefix to "nrd" from "sgi".

Signed-off-by: Rohit Mathew <Rohit.Mathew@arm.com>
Change-Id: I01a7fd6ad7e56e5690dc30adc0680f25ef450972
5 files changed