Meetings of the board of directors shall be held upon such notice as the bylaws may prescribe. Attendance of a director at any meeting shall constitute a waiver of notice of such meeting except where a director attends a meeting for the express purpose of objecting to the transaction of any business because the meeting is not lawfully called or convened. Unless provided otherwise in the articles of incorporation or the bylaws, neither the business to be transacted at, nor the purpose of, any regular or special meeting of the board of directors need be specified in the notice or waiver of notice of such meeting, except that no special meeting of directors may remove a director under Section 108.35(b) of this Act unless written notice of the proposed removal is delivered to all directors at least twenty days prior to such meeting.