systemc: Fix some paths in the tlm SCons(script|struct).

The code in util/systemc was moved into a subdirectory recently. The
scons support here referred to it, and needed to be updated.

Change-Id: Ib457d9bdafb2bba5058e6ecf99b9d33b3072c5b3
Reviewed-on: https://gem5-review.googlesource.com/c/13955
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>

misc: Reorder sources in util/tlm and rewrite build scripts

* Use one SConstruct to build everything instead of one SConstruct for
each example.
* Introduce SConscripts for sub-directories.
* Build in 'build' instead of the source tree.
* Build and link to SystemC from the ext/systemc directory. This
ensures that SystemC does not need to be installed on the host and
avoids possible issues caused by an incompatible SystemC build.
* Update the README and add some minor fixes

Change-Id: I641ed94f542626864fb7af499ad1be8fd4ad929f
Reviewed-on: https://gem5-review.googlesource.com/3480
Reviewed-by: Matthias Jung <jungma@eit.uni-kl.de>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>