insert --config=bp2build before -- in b args

Bazel also supports `--foo bar` as another way to mean `--foo=bar`,
The current implementation would accidentally catch that and make it
`--foo --config=bp2build bar`. This CL instead requires that additional
arguments to the target come after a `--`.

E.g.
```
b run --args-for-bazel //foo -- --args-for-foo
```

Test: b build //build/bazel/scripts/difftool:difftool
Test: b run //build/bazel/scripts/difftool/difftool.py --level=FINE /tmp/legacyBuildFiles /tmp/bazelBuildFiles --file_type=object
Change-Id: I534caab04e4c919d4e7b6dc83b8a88e020626b18