config: Add a default system disk image for SPARC FS.

config: Add a default system disk image for SPARC FS.

When the change below removed the hard coded disk name for the SPARC FS
configuration, it broke the regression which had not specified a disk name.
This change adds a default disk name so that the regression will continue to
work like it used to, but preserving the effect of this other change.

    commit 86a25bbcee88f6e69299867b6264885d738f636e
    Author: Jakub Jermar <jakub@jermar.eu>
    Date:   Tue Jul 19 09:52:46 2016 -0500

        config: Allow SPARC FS image to be specified on the command line

Change-Id: Ieb317b2bf573a4f2fc435d34cccd1f246c28d84c
Reviewed-on: https://gem5-review.googlesource.com/2645
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>