CheckerCPU: Make CheckerCPU runtime selectable instead of compile selectable

Enables the CheckerCPU to be selected at runtime with the --checker option
from the configs/example/fs.py and configs/example/se.py configuration
files.  Also merges with the SE/FS changes.

src/cpu/ozone/SConscript

@@ -53,6 +53,5 @@ if 'OzoneCPU' in env['CPU_MODELS']:
 
     CompoundFlag('OzoneCPUAll', [ 'BE', 'FE', 'IBE', 'OzoneLSQ', 'OzoneCPU' ])
 
-    if env['USE_CHECKER']:
-        SimObject('OzoneChecker.py')
-        Source('checker_builder.cc')
+    SimObject('OzoneChecker.py')
+    Source('checker_builder.cc')