XIA: add initial U4ID principal

This principal is used to tunnel XIP packets in UDP/IPv4 packets. This
commit adds entries in the appropriate Makefiles and adds a new
Makefile, Kconfig, and main file for the U4ID principal.

Functions are added for loading and unloading the module, as well as
initializing and freeing an instance of U4ID in u4id_net_init and
u4id_net_exit, respectively.
XIA/U4ID: initialize UDP socket for tunneling
XIA/U4ID: add tunneled packet handler
XIA/U4ID: add skeleton for XIA delivery function
XIA/U4ID: fix formatting
XIA/U4ID: add action for local U4IDs
XIA/U4ID: add code for main U4IDs
XIA/U4ID: fix dst input/output functions