[Bug] Fix some errors not caught on embedded builds

[laurensvalk]

I made this mistake earlier today:

    pbio_error_t err;
    while (pbio_port_get_servo(port, &type_id, &self->srv) == PBIO_ERROR_AGAIN) {
        mp_hal_delay_ms(1);
    }
    pb_assert(err);

Which should have been:

    pbio_error_t err;
    while ((err = pbio_port_get_servo(port, &type_id, &self->srv)) == PBIO_ERROR_AGAIN) {
        mp_hal_delay_ms(1);
    }
    pb_assert(err);

This raises the correct warning as error when building virtualhub but still passed on embedded hubs.

error: ‘err’ may be used uninitialized [-Werror=maybe-uninitialized]

We might be missing other warnings, so it would be good to make the embedded builds equally strict.