ruby: add RUBY sticky option that must be set to add ruby to the build

Default is false

src/mem/SConscript

@@ -34,7 +34,9 @@ SimObject('Bridge.py')
 SimObject('Bus.py')
 SimObject('MemObject.py')
 SimObject('PhysicalMemory.py')
-SimObject('RubyMemory.py')
+
+if env['RUBY']:
+    SimObject('RubyMemory.py')
 
 Source('bridge.cc')
 Source('bus.cc')
@@ -45,7 +47,9 @@ Source('physical.cc')
 Source('port.cc')
 Source('tport.cc')
 Source('mport.cc')
-Source('rubymem.cc')
+
+if env['RUBY']:
+    Source('rubymem.cc')
 
 if env['FULL_SYSTEM']:
     Source('vport.cc')