tlm: Don't set SystemC time resolution

Some simulators already set the time resolution of SystemC. By coupling
gem5 by means of SystemC with an other tool this can lead to problems:
When the resolution is set twice the simulation will not work.
Therefore, the line is changed to an assertion that checks if the time
resolution of the SystemC simulation is set to gem5's value of 1ps.

Change-Id: I8aafab9dd593eb4697a3c7f69ce181fd9cdd05c4
Reviewed-on: https://gem5-review.googlesource.com/4520
Reviewed-by: Paul Rosenfeld <prosenfeld@micron.com>
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>