config: Add --memchecker option

config: Add --memchecker option

This patch adds the --memchecker option, to denote that a MemChecker
should be instantiated for the system. The exact usage of the MemChecker
depends on the system configuration.

For now CacheConfig.py makes use of the option, adding MemCheckerMonitor
instances between CPUs and D-Caches.

Note, however, that currently this only provides limited checking on a
running system; other parts of the system, such as I/O devices are not
monitored, and may cause warnings to be issued by the monitor.