Convert license tags to SPDX

Also clarifies provenance of code in `apps/fdtgen/fdt_utils.c`

Signed-off-by: Gerwin Klein <gerwin.klein@data61.csiro.au>

Refactor camkes simulation tests to use CTest

CTest allows the tests to be better integrated with the project build
configuration. This makes maintaining and adding tests easier. It also
makes it possible to run the tests as part of a development workflow
with a near-instant incremental rebuild instead of having to configure
and build a full project from scratch.

Port example CAmkES applications to cmake

Adds a cmake build definition for each of the 'apps' and provides a small wrapper
CMakeLists.txt for setting common options and picking the application to build.