Introduce `b` into envsetup.sh.

`b` is a function that combines:

1. Integrated generation of a synthetic bazel workspace, containing
symlinks to BUILD and bzl files alongside symlinks to the source tree.
This is the --package_path of the bazel build.
2. Running the Bazel build itself with b's entire argv.

A user accustomed to typing `bazel build <targets>` would
now type `b build <targets>`.

Test: source build/envsetup.sh; b build //bionic/...; b cquery --output=label_kind //bionic/...
Fixes: 188490434
Change-Id: I36e366108b024c09945d764a1115786658e03681