List all available package upgrades.

    CLI Example:

    .. code-block:: bash

        salt '*' pkg.list_upgrades

def list_upgrades(refresh=True, **kwargs):  # pylint: disable=unused-argument
    '''
    List all available package upgrades.

    CLI Example:

    .. code-block:: bash

        salt '*' pkg.list_upgrades
    '''
    ret = {}
    if salt.utils.data.is_true(refresh):
        refresh_db()

    cmd = ['opkg', 'list-upgradable']
    call = __salt__['cmd.run_all'](cmd,
                                   output_loglevel='trace',
                                   python_shell=False)

    if call['retcode'] != 0:
        comment = ''
        if 'stderr' in call:
            comment += call['stderr']
        if 'stdout' in call:
            comment += call['stdout']
        raise CommandExecutionError(comment)
    else:
        out = call['stdout']

    for line in out.splitlines():
        name, _oldversion, newversion = line.split(' - ')
        ret[name] = newversion

    return ret