webos-image: Fix rootfs_set_image_name comments

Change ${sysconfdir} in comments to ${webos_sysconfdir} as it is in the
code itself

Open-webOS-DCO-1.0-Signed-off-by: Janne Hurskainen <janne.hurskainen@palm.com>

Change-Id: I0cd4368f714f2a1d1b8b3fda8d5a9d06308dc267
Reviewed-on: https://g2g.palm.com/1785
Reviewed-by: Janne Hurskainen <janne.hurskainen@palm.com>
Tested-by: Janne Hurskainen <janne.hurskainen@palm.com>
Reviewed-by: Build Verification
Reviewed-by: Keith Derrick <keith.derrick@lge.com>
Tested-by: Keith Derrick <keith.derrick@lge.com>
Reviewed-by: Susan Montooth <susan.montooth@lge.com>