misc: fix build failure in cxx_config example

Fix a missing header in the cxx_config example which is used as a
simple example of using libgem5.so without python.

Change-Id: I758bfe42ba735ce0c7eaedd49b94a130e3bd21e3
Reviewed-on: https://gem5-review.googlesource.com/3000
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-by: Matthias Jung <jungma@eit.uni-kl.de>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>

sim: Update C++ config example to match SystemC example

Update the use of the drain manager, and checkpointing to
match changes to gem5 since the example was written.

base: remove Trace::enabled flag

The DTRACE() macro tests both Trace::enabled and the specific flag. This
change uses the same administrative interface for enabling/disabling
tracing, but masks the SimpleFlags settings directly. This eliminates a
load for every DTRACE() test, e.g. DPRINTF.

config: Fix checkpoint restore in C++ config example

This patch fixes the checkpoint restore option in the example of C++
configuration (util/cxx_config).

The fix introduces a call to config_manager->startup() (which calls startup
on all SimObjects managed by that manager) to replicate the loop of
SimObject::startup calls in src/python/m5/simulate.py::simulate guarded by
need_startup. As util/cxx_config/main.cc is a C++ analogue of
src/python/mt/simulate.py, it should make a similar set of calls.

config: Add the ability to read a config file using C++ and Python

This patch adds the ability to load in config.ini files generated from
gem5 into another instance of gem5 built without Python configuration
support. The intended use case is for configuring gem5 when it is a
library embedded in another simulation system.

A parallel config file reader is also provided purely in Python to
demonstrate the approach taken and to provided similar functionality
for as-yet-unknown use models. The Python configuration file reader
can read both .ini and .json files.

C++ configuration file reading:

A command line option has been added for scons to enable C++ configuration
file reading: --with-cxx-config

There is an example in util/cxx_config that shows C++ configuration in action.
util/cxx_config/README explains how to build the example.

Configuration is achieved by the object CxxConfigManager. It handles
reading object descriptions from a CxxConfigFileBase object which
wraps a config file reader. The wrapper class CxxIniFile is provided
which wraps an IniFile for reading .ini files. Reading .json files
from C++ would be possible with a similar wrapper and a JSON parser.

After reading object descriptions, CxxConfigManager creates
SimObjectParam-derived objects from the classes in the (generated with this
patch) directory build/ARCH/cxx_config

CxxConfigManager can then build SimObjects from those SimObjectParams (in an
order dictated by the SimObject-value parameters on other objects) and bind
ports of the produced SimObjects.

A minimal set of instantiate-replacing member functions are provided by
CxxConfigManager and few of the member functions of SimObject (such as drain)
are extended onto CxxConfigManager.

Python configuration file reading (configs/example/read_config.py):

A Python version of the reader is also supplied with a similar interface to
CxxConfigFileBase (In Python: ConfigFile) to config file readers.

The Python config file reading will handle both .ini and .json files.

The object construction strategy is slightly different in Python from the C++
reader as you need to avoid objects prematurely becoming the children of other
objects when setting parameters.

Port binding also needs to be strictly in the same port-index order as the
original instantiation.