Adding submit or update button to custom metabox?

Don’t know if i agree with EarnestoDev answer which is more of an opinion then an answer based on facts and not true in all cases, since you can use jQuery to trigger
the submit event when a different element is clicked, so just add this js code once

<script>
jQuery('.metabox_submit').click(function(e) {
    e.preventDefault();
    jQuery('#publish').click();
});
</script>

and in each metabox add:

<input type="submit" class="metabox_submit" value="Submit" />