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>

misc: Clean up and complete the gem5<->SystemC-TLM bridge [4/10]

The current TLM bridge only provides a Slave Port that allows the gem5
world to send request to the SystemC world. This patch series refractors
and cleans up the existing code, and adds a Master Port that allows the
SystemC world to send requests to the gem5 world.

This patch:
* Move common code of the example to a common directory. Move the cli
* parsing from the SimControl module to a separate example object. Add
* comments describing the Gem5SimControl module.

Testing Done: Examples compile and run.

Reviewed at http://reviews.gem5.org/r/3695/

Signed-off-by: Jason Lowe-Power <jason@lowepower.com>