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
diff --git a/run_config/fvp-linux.sgi b/run_config/fvp-linux.sgi
index 467602b..dd3250c 100644
--- a/run_config/fvp-linux.sgi
+++ b/run_config/fvp-linux.sgi
@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
 #
-# Copyright (c) 2019-2023, Arm Limited. All rights reserved.
+# Copyright (c) 2019-2024, Arm Limited and Contributors. All rights reserved.
 #
 # SPDX-License-Identifier: BSD-3-Clause
 #
@@ -9,8 +9,8 @@
 	image="kernel" type="sgi-busybox" get_boot_image
 	image="initrd" type="sgi-ramdisk" get_boot_image
 
-	url="$sgi_prebuilts/grub-busybox.img" saveas="busybox.bin" fetch_file
-	url="$sgi_prebuilts/ramdisk-busybox.img" saveas="ramdisk.bin" fetch_file
+	url="$nrd_prebuilts/grub-busybox.img" saveas="busybox.bin" fetch_file
+	url="$nrd_prebuilts/ramdisk-busybox.img" saveas="ramdisk.bin" fetch_file
 
 	archive_file "busybox.bin"
 	archive_file "ramdisk.bin"