Add `--deps build` option to `bst checkout`

As discussed in
https://mail.gnome.org/archives/buildstream-list/2018-September/msg00064.html,
add `--deps build` option to `bst checkout`. This will allow users to
checkout the all build dependencies of a given element using a single
command.

- _frontend/cli.py: Add `--deps build` option for `bst checkout`.

- element.py: Support `deps='build'` in Element._prepare_sandbox().

- tests/frontend/buildcheckout.py: Ensure `--deps build` works as
  expected.