Fix firstboot fs mount logic

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.

Reviewed by:	imp
MFC after:	3 days
Differential Revision:	https://reviews.freebsd.org/D14226