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/run_config/qemu-linux.rootfs b/run_config/qemu-linux.rootfs
index fe5b4d1..982e6c1 100644
--- a/run_config/qemu-linux.rootfs
+++ b/run_config/qemu-linux.rootfs
@@ -1,11 +1,11 @@
 #!/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
 #
 
 fetch_tf_resource() {
-	kernel_type="qemu-busybox" get_kernel
+	image="kernel" type="qemu-busybox" get_boot_image
 	url="$rootfs_url" filename="rootfs.bin" fetch_and_archive
 }