download: Use shallow clones for big projects

Downloading coreboot and U-Boot takes quite the disk space and bandwith.
We don't need to download entire repos, only the revisions that we are
interested in.

Use the --depth=1 option to only download the files we need. Since the
initial clones may not have our target revision, always try to fetch it.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>