Kata OS

This is a CAmkES project that assembles the entire Kata OS. It exists outside the seL4 source trees, since it contains code not intended to go to upstream seL4.

This uses the standard CAmkES build system by symlinking CMakeLists.txt. It also symlinks settings.cmake, and so retains the notion of "apps," which enables the build system to switch which assembly it builds using the CAMKES_APP CMake cache value. Kata OS just has one app, system.