Add rxgk boilerplate

Error table, protocol description, and stubs for the security object
routines, with header to declare them.

The public header rxgk.h currently only contains the security level enum
and includes the RPC interface and com_err code headers.

Change-Id: I7893f78119bb4aef12112cc1e51e1ec69de326c2
Dummy Makefile for rxgk

Change-Id: I21c37787eeafb9bf99731f8bbe5d54dcab42c8bd
Hook rxgk directory up to the build

Change-Id: I8a42d14dd494fac10a999120e53045302a2e51d1