MFC r307826,r328949: Firstboot fs mount changes

MFC r307826: Use checkyesno instead of rolling my own..

MFC r328949: Fix firstboot fs mount logic

The firstboot logic has an error which causes the filesystem to be
mounted readonly even though root_rw_mount=YES. This fixes the error to
ensure that the root filesystem is mounted rw as expected after the run
of the firstboot scripts.