upstart: Add ROOTFS_READ_ONLY variable

:Release Notes:
Add the ROOTFS_READ_ONLY variable added by Yocto 1.4 to /etc/default/rcS
to Upstart's rcS. This variable indicates whether the rootfs has been
built to be read-only.

:Detailed Notes:

:Testing Performed:
1) MiniBAT
2) This change was tested as part of the full solution for read-only
   rootfs.

:QA Notes:

:Issues Addressed:
[GF-6589] Update scripts and programs for read-only rootfs

Open-webOS-DCO-1.0-Signed-off-by: Iyad Qumei <iyad.qumei@lge.com>
Change-Id: I6826a6069b09db4a94db49cbda852509bfbdb534
Reviewed-on: https://g2g.palm.com/2872
Reviewed-by: Build Verification
Reviewed-by: Iyad Qumei <iyad.qumei@lge.com>
Tested-by: Iyad Qumei <iyad.qumei@lge.com>
Reviewed-by: Ed Chejlava <ed.chejlava@lge.com>
Reviewed-by: Susan Montooth <susan.montooth@lge.com>