Git clone of the kernel takes quite a long time. Tried --depth=1. Needed the
following code (which doesn't quite replace the existing stuff).

# Seeing if need full checkout to generate our version string.
# Doesn't seem trivial unfortunately. git describe doesn't work.
# For dev, suggest using a local checkout to clone (near instant).
hash = self.x86_64.cmd("git rev-parse --short HEAD",
                       "get latest git short form hash")
tag = self.x86_64.cmd("git for-each-ref refs/tags --sort=-taggerdate "
                      "--format='%(refname)' --count=1",
                      "get latest git tag")

Git clone of a local directory is basically instant, so went with that for dev
work. No reason why this shouldn't be extended to an NFS mount or similar
in the cluster. You can git pull <parent URL> after the initial clone to get
any changes that haven't made it to the local server.

Leaving the toolchain cached and with a local clone, build on my inexpensive
AMD 4 core box took 3 minutes. This excludes most network access, but still has
the bzr checkout of the builddeb script from the Linaro branch + rest of the
repo.

There is no reason to download build tools every time, they could be cached.

Is there a reason to make clean? We have a clean checkout anyway...