The transactions of any meeting of the board, however called and noticed or wherever held are as valid as though undertaken at a meeting duly held after regular call and notice if a quorum is present and if, either before or after the meeting, each of the directors not present signs a written waiver of notice, a consent to holding the meeting, or an approval of the minutes thereof. All such waivers, consents, and approvals shall be filed with the corporate records or made a part of the minutes of the meeting. Waivers of notice or consents need not specify the purpose of the meeting.