Fix #560060 - Embedded "app.config" is not read when used inside a bundle.

Fix #560060 - Embedded "app.config" is not read when used inside a bundle.

	* mcs/class/System.Configuration/System.Configuration/InternalConfigurationHost.cs: Add new internal call to retrieve embedded application config. First
	check if an embedded configuration exists before looking up on the
	filesystem.

	* mono/metadata/icall-def.h: Add entry for retrieving embedded application
	configuration.

	* mono/metadata/icall.c: Add function to retrieve embedded application
	configuration if one has been registered with the runtime.