efi_loader: let the variable driver patch out the runtime

Our variable services are only provided at boottime. Therefore when
leaving boottime the variable function are replaced by dummy functions
returning EFI_UNSUPPORTED. Move this patching of the runtime table to the
variable services implementation. Executed it in ExitBootServices().

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/include/efi_loader.h b/include/efi_loader.h
index d30c4e8..8d75dde 100644
--- a/include/efi_loader.h
+++ b/include/efi_loader.h
@@ -327,6 +327,8 @@
 efi_status_t efi_init_obj_list(void);
 /* Initialize variable services */
 efi_status_t efi_init_variables(void);
+/* Notify ExitBootServices() is called */
+void efi_variables_boot_exit_notify(void);
 /* Called by bootefi to initialize root node */
 efi_status_t efi_root_node_register(void);
 /* Called by bootefi to initialize runtime */