config: Break out base options for usage with NULL ISA

This patch breaks out the most basic configuration options into a set
of base options, to allow them to be used also by scripts that do not
involve any ISA, and thus no actual CPUs or devices.

The patch also fixes a few modules so that they can be imported in a
NULL build, and avoid dragging in FSConfig every time Options is
imported.

configs/common/CpuConfig.py

@@ -70,7 +70,7 @@ def is_cpu_class(cls):
         return issubclass(cls, m5.objects.BaseCPU) and \
             not cls.abstract and \
             not issubclass(cls, m5.objects.CheckerCPU)
-    except TypeError:
+    except (TypeError, AttributeError):
         return False
 
 def get(name):