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
3 files changed