webos-image.bbclass: Support read-only rootfs when using Upstart 1.8

:Release Notes:
Added ROOTFS_READ_ONLY setting to /etc/init/SetInitEnv.conf since
/etc/default/rcS isn't used with Update 1.8 .

:Detailed Notes:
Also modified webos_read_only_rootfs_hook() to look for a
ROOTFS_READ_ONLY setting to correct in /etc/init/SetInitEnv.conf as well
as in /etc/default/rcS.

If both /etc/default/rcS and /etc/init/SetInitEnv.conf are not found,
then ROOTFS_READ_ONLY will not be properly passed to the init process,
and therefore this class file must be modified to remedy that issue.

NOTE: Name of the script SetInitEnv.conf will be changed by GF-12676.

:Testing Performed:
- Built squashfs image is done successfully using Upstart 1.8 with all
  the changes in GF-11551.
- MiniBAT (see GF-11551)

:QA Notes:

:Issues Addressed:
[GF-12738] squashfs image fails to build when using Upstart 1.8

Open-webOS-DCO-1.0-Signed-off-by: Wonhong Kwon <wonhong.kwon@lge.com>
Change-Id: I5394af77d10619c65d89fece66a1ed87ea01371f
Reviewed-on: https://g2g.palm.com/3005
Reviewed-by: Susan Montooth <susan.montooth@lge.com>
Tested-by: Susan Montooth <susan.montooth@lge.com>