Support context mapping at Subsystem level

BZ: 176942

Add a class (ConfigurableMappingElement) handling the mapping; to be derived by
all classes that can contain mapping.  ConfigurableMappingElement derives from
ConfigurableElement.  Until now, the only child of ConfigurableElement that
could have mapping was InstanceConfigurableElement (through TypeElement, that
contains all information related to its type).

Now, both InstanceConfigurableElement and Subsystem derive from
ConfigurableMappingElement.  That way, Subsystem XML elements can now have a
"Mapping" attribute, propagated to all Elements in the tree.  The Subsystem.xsd
schema is updated in that regard.

Only context mapping is relevant at the Subsystem-level (as opposed to
instantiation mapping).  If an instantiation mapping is set in the Subsystem,
it is silently ignored.

Change-Id: I3c09959bc5e3d18c5a4d354e498d69cc5489a247
Signed-off-by: David Wagner <david.wagner@intel.com>
Signed-off-by: Mattijs Korpershoek <mattijsx.korpershoek@intel.com>