sysvinit: add ROOTFS_READ_ONLY variable to rcS-default
This variable indicates whether the rootfs is intended to be read-only
or not. Changing this value from 'no' to 'yes' on a currently running
system with read-write rootfs and rebooting will give the user a working
system with read-only rootfs.
However, it is not suggested to change its value. Normally, if a read-only
rootfs is required, we should build an image with 'read-only-rootfs' image
feature.
