Load RuntimeConfig from plugins

Implements support for dynamically loading additional RuntimeConfig
and associated RuntimeParameter that are defined in a plugin.

Currently, the various RuntimeConfig's are hard coded in a list within
WA. This patch extends RuntimeParameterManager to use PluginLoader to
load RuntimeConfig classes and append to the hard coded list.

The implementation, as written, does not allow loading RuntimeConfig
from a plugin if it has the same name as one of the hard coded
RuntimeConfig. This is meant to prevent conflicts and unexpected
behavior.

wa/framework/target/runtime_parameter_manager.py

@@ -22,6 +22,7 @@ from wa.framework.target.runtime_config import (SysfileValuesRuntimeConfig,
                                                 CpuidleRuntimeConfig,
                                                 AndroidRuntimeConfig)
 from wa.utils.types import obj_dict, caseless_string
+from wa.framework import pluginloader
 
 
 class RuntimeParameterManager(object):
@@ -37,9 +38,16 @@ class RuntimeParameterManager(object):
 
     def __init__(self, target):
         self.target = target
-        self.runtime_configs = [cls(self.target) for cls in self.runtime_config_cls]
         self.runtime_params = {}
 
+        try:
+            for rt_cls in pluginloader.list_plugins(kind='runtime-config'):
+                if rt_cls not in self.runtime_config_cls:
+                    self.runtime_config_cls.append(rt_cls)
+        except ValueError:
+            pass
+        self.runtime_configs = [cls(self.target) for cls in self.runtime_config_cls]
+
         runtime_parameter = namedtuple('RuntimeParameter', 'cfg_point, rt_config')
         for cfg in self.runtime_configs:
             for param in cfg.supported_parameters: