The boards of county road commissioners are also authorized to expend funds either raised by general taxation or received from the distribution of motor vehicle funds for the purposes of the construction and/or maintenance of such forest highways. For the purposes of construction the counties’ share of such cost shall be such share as may be mutually agreed upon between the United States government, the state highway commissioner and the board of county road commissioners. The maintenance of any such road built under the provisions of this act shall be a direct obligation of the county in which the said road is located and shall be to all intents and purposes a part of the county road system of such county and shall so remain unless such road has been or may be hereafter lawfully established as trunk line highway. The maintenance and construction of trunk line highways through the United States forests shall be subject to the general provisions of law relating thereto.