feat: add functions for retrieving prebuilts

Add generic functions for retrieving prebuilt binaries. At the moment,
the platforms implement some flavour of these in their *_utils.sh files.
None of these implementations do anything particularly special, for this
reason, should be made generic so future platforms can re-use as much as
possible.

Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>
Change-Id: I16abf60e124fa9ef6e8f0016e7b8a6537f238e5e
diff --git a/qemu_utils.sh b/qemu_utils.sh
index 29b8e61..f5e5857 100644
--- a/qemu_utils.sh
+++ b/qemu_utils.sh
@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
 #
-# Copyright (c) 2022 Arm Limited. All rights reserved.
+# Copyright (c) 2023 Arm Limited. All rights reserved.
 #
 # SPDX-License-Identifier: BSD-3-Clause
 #
@@ -14,7 +14,7 @@
 default_model_dtb="dtb.bin"
 
 # QEMU Kernel URLs
-declare -A plat_kernel_list=(
+declare -A kernel_list=(
 	[qemu-busybox]="$tfa_downloads/linux_boot/Image.gz"
 )