util, tlm: Fix a memory error in the SCMasterPort class.

In the b_transport method of the SCMasterPort class, there is a check
which determines whether the packet being sent to gem5 should be
deleted once the call to sendAtomic returns. This was deleting the
packet if extension was *not* nullptr.

This check should delete the packet if the extension *is* nullptr. The
reasoning is that the extension will equal nullptr if there was no
gem5 packet in an extension and a new one needed to be allocated. If
there was an extension, ie if extension is not nullptr, then that's
where the packet came from which therefore doesn't belong to us. In
that case, we need to leave it alone and let its owner clean it up.

With the check reversed, this method will either leak allocated packets
it should delete, or delete packets it shouldn't that someone else will
likely try to use later.

Change-Id: I61578d910be6e5085b9fc0ddaa82468b1ac68578
Reviewed-on: https://gem5-review.googlesource.com/c/16949
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>

misc: Using smart pointers for memory Requests

This patch is changing the underlying type for RequestPtr from Request*
to shared_ptr<Request>. Having memory requests being managed by smart
pointers will simplify the code; it will also prevent memory leakage and
dangling pointers.

Change-Id: I7749af38a11ac8eb4d53d8df1252951e0890fde3
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/10996
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.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>