Remove everything from this branch to save drive space

There currently doesn't seem to be a way to have a local manifest just
remove a single directory.  A way to get around this is to check out
an empty repository, so create one.  This allows us to remove several
copies of the kernel that are currently pulled into the tree, saving up
to 7GB.

BUG=None
BRANCH=None
TEST=create a local manifest and add the following lines:
<manifest>
  <extend-project path="src/third_party/kernel/v3.8"
           name="chromiumos/third_party/kernel"
           revision="refs/heads/empty" />
  <extend-project path="src/third_party/kernel/v3.10"
           name="chromiumos/third_party/kernel"
           revision="refs/heads/empty" />
  <extend-project path="src/third_party/kernel/v3.14"
           name="chromiumos/third_party/kernel"
           revision="refs/heads/empty" />
  <extend-project path="src/third_party/kernel/v3.18"
           name="chromiumos/third_party/kernel"
           revision="refs/heads/empty" />
  <extend-project path="src/third_party/kernel/v4.4"
           name="chromiumos/third_party/kernel"
           revision="refs/heads/empty" />
  <extend-project path="src/third_party/kernel/v4.14"
           name="chromiumos/third_party/kernel"
           revision="refs/heads/empty" />
  <extend-project path="src/third_party/kernel/v4.14-mistral"
           name="chromiumos/third_party/kernel"
           revision="refs/heads/empty" />
  <extend-project path="src/third_party/kernel/v4.14-gw"
           name="chromiumos/third_party/kernel"
           revision="refs/heads/empty" />
  <extend-project path="src/third_party/kernel/v4.19"
           name="chromiumos/third_party/kernel"
           revision="refs/heads/empty" />
  <extend-project path="src/third_party/kernel/v4.19-lakitu"
           name="chromiumos/third_party/kernel"
           revision="refs/heads/empty" />
</manifest>

Change-Id: Ied15a70693bd7b23657fd356acc17d9fd3dbd9c6
Signed-off-by: Martin Roth <martinroth@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/1881817