Add build-camkes.sh script to cross-build build public camkes.

This script builds the public seL4 camkes setup using the Rust rootserver
and related bits. The resulting image can be run to sanity-check the
Rust support on various platforms.

This is mostly used for portability testing and checking upstream compat.
Long-term this may be used to demonstrate public use of the Rust bits
of the system.

Change-Id: Ifcc05f89ef8f305b5d2639e1d947a07a02e9b750