travis/gitlab/azure: Use -W to avoid warnings check

We can use the -W flag to tell buildman to ignore warnings. Since we also
have -E defined, compiler warnings are promoted to errors, so they will
still cause a failure. But migration warnings of the form:

    ===================== WARNING ======================
    This board does not use CONFIG_DM. CONFIG_DM will be
    compulsory starting with the v2020.01 release.
    Failure to update may result in board removal.
    See doc/driver-model/migration.rst for more info.

will now be ignored.

Signed-off-by: Simon Glass <sjg@chromium.org>
Fixes: 329f5ef5 (travis.yml: run buildman with option -E)
Reviewed-by: Tom Rini <trini@konsulko.com>