Added a build option for GEM5 to enable database features (default: off).
Added a build option for GEM5 to enable database features (default: off).

To use:
  scons build/X86/gem5.opt --use_db

As usual, this will require the user to define the MYSQL_HOME env variable.

Change-Id: I30d8395b9ff24247af3ba006ea360d07bb59f8a2

andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin