webos-image.bbclass: Optionally prepare rootfs to be read-only

:Release Notes:
Make webOS-specific adjustments to various parts of the rootfs in
preparation for read-only mounting when "read-only-rootfs" appears in
IMAGE_FEATURES.

: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-6590] Mount directory trees that must be writable on writable
          partitions

Open-webOS-DCO-1.0-Signed-off-by: Iyad Qumei <iyad.qumei@lge.com>
Change-Id: I9ffcb268e6649ba753d98bb020343205616c63bd
Reviewed-on: https://g2g.palm.com/2803
Reviewed-by: Build Verification
Reviewed-by: Iyad Qumei <iyad.qumei@lge.com>
Tested-by: Iyad Qumei <iyad.qumei@lge.com>
Reviewed-by: Susan Montooth <susan.montooth@lge.com>